Abstract: The tongue sole, Cynoglossus semilaevis is a Chinese inshore species with a great potential for aquaculture. In spite of great commercial interest, there is limited information about the biology, nutritional requirements or techniques for intensive rearing of larvae of this species. An estimation of the utilization of endogenous nutrients from the yolksac that occurs during embry onic and early larval development can be a useful approach to the study of the nutritional requirements of fish larvae. In order to estimate the metabolic characteristics and nutritional requirements of Tongue sole first feeding larvae, the changes in amino acid and fatty acid content that occur during the rapid development of the embryonic and yolk sac larval stages were examined. Eggs obtained from captive broodstock which spawned naturally, and larvae used were hatchery reared from the same batch. Eggs were incubated at 19) 22 e in incubation baskets held in a hatchery tank with gentle aeration and a continuous flow of 014L/min. After hatching, larvaewere reared in 500 L indoor circular tanks at (21 ? 1) e temperature and 30) 32 g/L salinity.The composition of amino acid and fatty acid in eggs and larvaewere examined with gas chromatography and automatic amino acid analyser. The results showed that the composition of total amino acids changed significantly from egg to yolk sac larvae, but from yolk sac larvae to first2feeding larvae, it showed minor compositional changes. The composition of total, essential amino acids (EAAs) in first2feeding larvae was closely correlated to the composition in rotifers, which suggested no major dietary imbalances of EAAs. High concentrations of free amino acids (FAAs) were found in tongue sole eggs, i1e. 139 mg/g dryweight (DW) and 2213% of total amino acids. Concentrations dropped significantly in yolk sac larvae (316 mg/g) and no further changed in lar vae at first feeding (215 mg/g) . These changes suggested amajor metabolic role of FAAs during early ontogeny of tongue sole.Total saturated fatty acids content significantly decreased from fertilized egg to yolk sac larvae, but no significant difference was found between the contents of yolk sac larvae and first feeding larvae. Total monounsaturated fatty acids and total polyunsaturated fatty acids contents were not significantly different among different developmental stages. Eicosapentaenoic acid (20: 5n23) and docosahexaenoic acid (22: 6n23) significantly decreased from yolk sac larvae to first feeding larvae, while arachidonic acid (20:4n26) was specifically retained throughout development, which suggested a requirement for long chain polyunsaturated fattyacids such as 20: 5n23, 20:4n26 and 22: 6n23.