Summer(Aug.) and winter(Dec.) silver carp,Hypophthalmichthys molitrix were studied.Because chymotrypsin catalyzes the hydrolysis of peptide bonds adjacent aromatic amino acids,it can be used to detect the difference in the protein internal structure combining with SDS-PAGE technique.The objective of the study is to characterize the structural stability and thermal stability of myosin from summer and winter silver carp by using chymotryptic digestion.The results showed that there was a longer subfragment of myosin S-1 produced by chymotrytic digestion for winter myosin,compared with that of summer one.If myosin was cleaved into HMM and LMM,the winter sample was liable to be cleaved into a shorter fragment as 135 ku HMM,while summer myosin still produced long 165 ku HMM even if digested at high temperatures(30 ℃).These results indicated the instability of winter myosin.Heating summer and winter myofibril at different temperatures,and then measuring the remaining ATPase activity and the amount of S-1 produced by chymotyptic digestion,it was found that the decreasing rate of ATPase activity and S-1 produced was similar to decreasing temperature.This indicated that S-1 produced by chymotryptic digestion only originated from native myosin.Furthermore,as the denaturation temperature for summer myosin was 6 ℃ higher than winter myosin,it indicated that summer myosin was more stable than winter one.This paper introduced chmotrypsin digestion as a very useful method to study the biochemical properties of muscle proteins such as myosin,which is a very important protein for gel formation.The information will be very useful for producing surimi from freshwater fish species in different seasons.