Abstract: 1) Changes of biochemical, composition in muscles of silver carp (Hypophthalmichys molitrix (Cuv. et Val.)) and bighead (Aristichthys nobilis (Rich)) with respect to different body-lengths were studied.Samples were collected from lake Donghu, Wuhan, in late autumn 1979 and early winter 1980. The body-length of fishes were as follows: 61 silver carp of 11.44—74.21 cm (14.16——7551.77g) and 64 bighead of 12.02——68.30 cm (32.06—5942.13g). The biochemical composition of muscles was analysised for fishes of different body-lengths.The results of analysis were: (1)The water content of muscle in silver carp is 60.29—80.34%, protein—15.32—17.18%, fat—2.00—20.84%, non-nitrogen extract—0.17—1.71% and ash—1.04—1.39%. The water content of muscle in bighead is 73.17—83.13%, protein—14.75—18.53%, fat—0.96—7.8%, non-nitrogen extract—0.08—1.33% and ash—1.05—1.29% With the increase of body-length the water content in muscle decreases and, on the contrary, the fat content of muscle increases. The contents of other substances are not apparently correlated with body-length. (2) The water content in muscle of bighead of the same body-length is higher than that of silver carp, while the fat content, lower. The content of other substances are about the same.2) Samples for the study of seasonal Variation of biochemical composition of 1+ and 2+age group of 55 silver carp and 53 bighead were collected form February to December 1980 in lake Donghu, Wuhan.The results are as follows: (1) variations of biochemical composition of muscle in both age groups demonstrate that the water content decreases and protein and fat content-increase gradually as the fishes grow season by season. Chan-ges of non-nitrogen extract and of ash content however, are insignificant. (2) water content in 1+ fish is higher than that in 2 + fish, protein, and fat in 1~+ fish are lower than that in 2+ fish.