Five species in the genus Thalia of the family Salpidae are described: Thalia cicar van Soest, 1973, Thalia democratica (Forskal, 1775), Thalia orientalis Tokioka, 1937, Thalia rhomboides (Quoy and Gaimard, 1824),and Thalia sibogae van Soest, 1973. All of these species are new to the Korean fauna. A key to the Korean Thalia species is provided. The species of the family Salpidae are free-swimming and filter-feeding marine holoplanktons. The life-cycles of the species alternate between two forms, an asexually budding solitary stage and a sexually reproducing aggregate stage.Forty species in 13 genera are currently known (Chihara and Murano, 1997; Esnal and Daponte, 1999), whereas in Korea three species in three genera have been recorded (Rho, 1967;Kim et al., 2010a, 2010b). Identification of salpid species is based mainly on the arrangement of body muscles. For identification of species in the genus Thalia , the various projections of the test, the transparent outer surface of body, are also important (van Soest, 1973). In a series of taxonomic studies on planktonic tunicate, five species in the genus Thalia were newly identified from Korean waters. This paper aims to re-describe these species which are new to the Korean fauna, with a key to Korean Thalia species. The specimens were collected with an IKMT net of 417 μm mesh. They were fixed and stored in 4% buffered formalin or 80% ethyl alcohol, and then examined with a stereoscopic microscope (Leica MZ16 with AxioCam HRc). The abbreviation M indicates the muscle band. The body length was measured by using objective and ocular micrometers.Solitary zooids were measured from the terminal of the oral opening to the tip of midventral projections, whereas aggregate zooids were measured from the terminal of the oral opening to the atrial opening. Materials examined in this study have been deposited in the Seodaemun Museum of Natural History. Diagnosis. Solitary zooid: Test cylindrical, thickened ventrally around nucleus, with two long posterior projections.Oral opening terminal, atrial opening postero-dorsal. Five body muscles, MI to MIV continuous dorsally and ventrally,MV interrupted ventrally. MI to MIII and MIV to MV contiguous or fused in mid-dorsal line. Intermediate muscle interrupted dorsally and converge to MI. Dorsal tubercle simple and small, separated from ganglion with short distance. Branchial septum slender, extending from ganglion to gut. Endostyle thin, extending from behind oral to posterior ventrally.Gut forming a compact nucleus, stolon coiled around it. Aggregate zooid: Body oval, rounded or pointed posteriorly.Both openings dorsal. Five body muscles continuous dorsally while interrupted ventrally. MI to MIII and MIV to MV fused in mid-dorsal line. Gut forming a compact nucleus. Testis arranged in 4 lobes. Ovary and embryo underneath MIV. Materials examined . 5 solitary zooids, Korea strait, 32°29′ N, 127°29′E, 4 May 2001; 1 aggregate zooid, South Sea, 35° 20′N, 130° 12′E, 15 Oct 2001; 3 solitary zooids and 1 aggregate zooid, East Sea, 37°00′N, 129° 30′E, 23 Sep 2003. Description. Solitary zooid : 4-15 mm long, excluding posterior projections ( Fig.1 A , B ). Body flabby, relatively short( Fig.1 A , B ). Test thick and smooth ( Fig.1 A , B ). Atrial palps large, bifurcate ( Fig.1 C ). Lateral projections well developed and posterior projections relatively short ( Fig.1 A , B ). Medioventral projections almost same length, well developed( Fig.1 B ). Echinate restricted to projections and surrounding oral opening ( Fig.1 A - C ). Body muscles thin ( Fig.1 A ). Aggregate zooid ( Fig. 1D ): 5-10 mm long, body oval with round posterior. Attachment organs variable in number and protruding beyond test wall left posterior. Distribution. Pacific Ocean, Atlantic Ocean, and Mediterranean Sea. Materials examined. 1 aggregate zooid, East China Sea, 31° 05′N, 126° 49′E, 4 May 2001; 7 solitary zooids and 2 aggregate zooids, Korea strait, 32° 29′N, 127° 29′E, 4 May 2001;6 solitary zooids, Korea strait, 33° 29′N, 127° 45′E, 5 May 2001; 14 solitary zooids and 13 aggregate zooids, Korea strait, 33° 00′N, 127° 30′E, 5 May 2001; 11 solitary zooids and 58 aggregate zooids, East China Sea, 24° 40′N, 125° 00′E,7 May 2001; 1 aggragate zooid, South Sea, 34° 05′N, 127° 96′E, 4 Oct 2007; 6 aggregate zooids, South Sea, 34° 05′N,127° 96′E, 1 Sep 2008. Description. Solitary zooid: 2-8 mm long, excluding posterior projections ( Fig.2 A ). Body oval or elongated cylindrical( Fig.2 A ). Test smooth ( Fig.2 A ). Atrial palps simple and straight, not bifurcated ( Fig.2 C ). Medioventral projections small and lower one longer than upper one ( Fig.2 B ). Lateral projections small. All projections echinate ( Fig.2 A - C ). Body muscles thin ( Fig.2 A ). Aggregate zooid ( Fig. 2D ): 1-8 mm long. Body oval with bluntly projected posteriorly. Test smooth. Attachment organs variable in number and protruding short beyond test wall left posterior. Nucleus bearing a projection posterior (nucleus projection). Distribution. Pacific Ocean, Atlantic Ocean, Indian Ocean,Mediterranean Sea, and Red Sea. Remarks. This species was reported previously in the “List of Animals in Korea” (The Korean Society of Systematic Zoology, 1997) under Salpa mucronata without any description or remarks. In the report, the Korean name “salpa” referring to North Korean name without any source was presented.Therefore, in this study, this species is reported as a new record to the Korean fauna with a new Korean name referring to the small body size of this species. Materials examined. 1 aggregate zooid, Korea strait, 32° 29′ N, 127° 29′E, 4 May 2001; 2 solitary zooids, Korea strait, 33° 00′N, 127° 30′E, 5 May 2001; 1 solitary zooid, Korea strait, 33° 59′N, 128° 17′E, 6 May 2001; 2 solitary zooids,South Korea, 35° 06′N, 130° 01′E, 12 Oct 2001; 6 solitary zooids and 3 aggregate zooids, East Sea, 38° 30′N, 129° 49′E,15 Oct 2001; 1 solitary zooid, East Sea, 36° 30′N, 130° 32′E, 15 Oct 2001; 65 solitary zooids and 10 aggregate zooids, South Korea, 35° 20′N, 130° 12′E, 15 Oct 2001; 6 solitary zooids and 38 aggregate zooids, East Sea, 37° 00′N, 129° 30′E,23 Sep 2002; 1 solitary zooid, Korea strait, 33° 59′E, 128° 17′N, 24 Sep 2002; 65 solitary zooids and 2 aggregate zooids,East China Sea, 29° 15′N, 125° 50′E, 24 Sep 2002; 13 solitary zooids, South Korea, 34° 57′N, 129° 22′E, 3 Dec 2002; 1 aggregate zooid, South Korea, 34° 05′N, 127° 96′E, 1 Sep 2008; 1 aggregate zooid, East Sea, 35° 46′21.54′′N, 129° 31′39.69′′E, 22 Oct 2010. Description. Solitary zooid: 3-14 mm long, excluding posterior projections ( Fig.3 A , B ). Body oval or cylindrical ( Fig.3A , B ). Test smooth ( Fig.3 A , B ). Atrial palps bifurcate ( Fig.3C ). Posterior projections very long that almost same length with trunk ( Fig.3 A , B ). Medioventral projections well developed and lower one longer than upper one ( Fig.3 B ). Lateral projections not exist. All projections echinate ( Fig.3 A - C ).Body muscles thin ( Fig.3 A , B ). Aggregate zooid ( Fig. 3D ): 1-13 mm long. Body oval with projected posteriorly. Test smooth. Attachment organs variable in number and protruding beyond test, especially right anterior and left posterior. Nucleus oval. Distribution. Pacific Ocean, Atlantic Ocean, and Indian Ocean. Materials examined. 17 solitary zooids and 14 aggregate zooids, Korea strait, 32°29′N, 127°29′E, 8 May 2001; 1 solitary zooid, South Korea, 35#176; 24′N, 129° 34′E, 3 Dec 2001;40 solitary zooids and 7 aggregate zooids, Korea strait, 32° 29′N, 127°29′E, 24 Sep 2002; 1 aggregate zooid, Korea strait, 33°59′N, 128°17′E, 24 Sep 2002; 46 solitary zooids and 12 aggregate zooids, Korea strait, 31°59′N, 127°29′E,2 Dec 2002; 1 aggregate zooid, South Korea, 34°05′N, 127°96′E, 1 Sep 2008; 2 solitary zooids, South Korea, 35°20′N,130°12′E, 15 Oct 2001; 1 solitary zooid and 1 aggregate zooid, East Sea, 38°30′N, 129°49′E, 15 Oct 2001; 1 solitary zooid and 1 aggregate zooid, Yeosu, 34°42′N, 127°40′E, Nov 2002; 1 aggregate zooid, 35°47′33.06′′N, 129°32′27.37′′E,22 Oct 2010. Description. Solitary zooid: 3-12 mm long, excluding posterior projections ( Fig.4 A , B ). Body elongated cylindrical( Fig.4 A , B ). All test echinate, especially projections ( Fig.4A - C ). Atrial palps bifurcate ( Fig.4 C ). Medioventral projections well developed and lower one longer than upper one( Fig.4 A , B ). Lateral projections well developed ( Fig.4 A , B ).Body muscles thick ( Fig.4 A , B ). Aggregate zooid ( Fig. 4D ): 1-2 mm long. Body pentagonal shape with sharp-edged with Echinated. Sharply pointed posterior.Test relatively hard. Attachment argans not protruding test well. Nucleus oval. Distribution. Pacific Ocean, Atlantic Ocean, and Indian Ocean. Materials examined. 9 solitary zooids, South Korea, 35° 20′N, 130° 12′E, 8 Sep 2002. Description. Solitary zooid: 7-11 mm long, excluding posterior projections ( Fig.5 A , B ). Body oval or cylindrical ( Fig.5A , B ). All test echinate, especially projections ( Fig.5 A , B ).Atrial palps bifurcate ( Fig.5 C ). Medioventral projections short, lower one longer than upper one ( Fig.5 B ). Lateral projections very short or not developed. Posterior projections relatively short ( Fig.5 A , B ). Body muscles very thick ( Fig.5A , B ). Distribution. Pacific Ocean and Indian Ocean. Remarks. Aggregate zooids of this species were not collected during this study. - Oval body with a projection at posterior end or not ∙∙∙∙∙∙∙ 2 2. Nucleus bearing a projection at posterior end (nucleus projection)∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ T. democratica - Nucleus oval without nucleus projection ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ 3 3. Body with round posterior end ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ T. cicar - Body with projected posterior end ∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙∙ T. orientalis