Scallop larvae showed similar shell growth when fed diets consisting of stationary phase cells of Isochrysis aff. galbana (clone T-iso) with logarithmic phase cells of Chaetoceros calcitrans or logarithmic phase cells of both species; however, significant differences were observed in the nutritional conditions of the larvae, segregated into two size classes. Total energy contents were higher in both sizes of larvae fed the diet with stationary phase cells of Isochrysis than in both sizes fed the logarithmic phase cells. Larger sized larvae fed either diet contained higher total energy reserves than their smaller counterparts, principally from levels of stored neutral lipids. Total lipids in larger larvae fed either diet consisted of lower levels of monoethylenic and n6-polyunsaturated fatty acids and higher levels of polyethylenic and n3-polyunsaturated fatty acids than lipids from corresponding smaller larvae, suggesting a requirement for the n3 family of fatty acids in developing larvae. Three diets, a binary diet of I. aff. galbana (T-iso) and C. calcitrans and two ternary diets composed of the binary diet, with added Tetraselmis suecica and with added Thalassiosira pseudonana were fed to larvae. Biochemical analyses performed on the diets and resultant larvae showed no correlation between protein or lipid content in the diets and the total energy reserves in the larvae. Incorporation of polyunsaturated fatty acids, 20: 5n3 and 22:6n3, into larval tissue was dependent on their proportion in the diet, with the ternary diet containing T. pseudonana providing the greatest amount of 20:5n3 acid. Of all diets, the ternary with T. pseudonana contained the highest level of carbohydrate and resulted in larvae with the highest nutritional condition. The high correlation between dietary carbohydrate and larval quality in this study illustrates the significant role carbohydrate plays in balancing the use of nutrients for synthesis as opposed to energy production.