Myofibrils were prepared from dorsal muscle of silver carp monthly from Jul 2003 to Jun 2004. Myofibrils of silver carp in the summer season were much more stable than those in the winter season as measured by (Ca-ATPase) thermal inactivation rate (kD) at 0.1 M KCl. The kD of myofibrils measured at 2 M KCl was referred as 1 for myosin without protection by F-actin. It was proved that myosin itself in summer season fish (summer-type) was very stable compared with that in winter season fish (winter-type). Winter-type myosin easily denatured upon storage in ice. Analysis of the thermal inactivation profiles of myosin monthly prepared demonstrated that myosins in December, January, February, and March were winter-type, and ones in June, July, August, and September were summer-type. It was also demonstrated that both winter-type and summer-type myosin at different ratios coexisted in the transit season (April, May, October, November), showing the breaking point in the inactivation profile. It was concluded that silver carp expresses 2 types of myosin with different kD seasonally and that the exchange of the expression completed in 2 mo.