During September-November 1977 ard May-July 1978, two cruises had been ma-de by the R/V Shi--Yan of Nan Hai Institute of Oceanology, Academia Sinica. Theareas of cruises were located in the central region of Nan Hai between 110°--118°Eand 12°--16°30′N. The specimens of 282 tuna fish larvae collected in these two cruisesimcluded yellowfin tuna, Thunnus albacares (Bonnaterre) (95 specimens); the albacore,Thunnus alalunga (Bonnaterre) (7); the big--eye tuna, Thunnus obesus (Lowe) (69); theblue-fin tuna, Thunnus thynnus (Linnaeus)(3); the longtail tuna, Thunnus tonggol(Bleeker)(2); Thunnus spp. (8); the skipjack tuna, Katsuwonus pelamis (Linnaeus)(65); the oceanic bonito, Euthynnus affinis (Cantor) (7); the yaito bonito, Euthynnusaffinis yaito kishinouye (14); the frigate mackerel, Auxis thazard (Lacepede) (3) andAuxis sp. (9). the composition of these species. the characteristics distribution and theirrelation to the temperature and salinity of surface sea waters are discussed in this paper.