Silver carp Hypopthalmichthys molitrix is eurythermal temperate fish, whose muscle is considered to express several types of myosin heavy chain (MYH) genes at different stages of its growth and to adjust to the environmental temperature. In this study, MYH genes expressed in the muscles of juvenile and adult silver carp were investigated. Five types of MYH cDNA clone were isolated from silver carp (H. molitrix) by RACE strategy using a set of fast-type MYH specific primers, and termed scMYH(F1), scMYH(F2), scMYH(F3), scMYH(F4) and scMYH(F5) in the order of their abundance in cDNA libraries constructed from fast skeletal muscles of adult silver carp. scMYH(F1), scMYH(F3) and scMYH(F5) showed high nucleotide sequence identities of 96, 98 and 96% to gcMYH(F30), gcMYH(F10) and gcMYH(FI), respectively, that encode MYHs predominantly expressed in fast skeletal muscle of grass carp (Ctenopharyngodon idella) acclimated to 30, 10 and 20 degrees C, respectively. scMYH(F2) and scMYH(F4) showed a high identity to A4-type MYH from rock cod (Notothenia coriiceps) slow skeletal muscle. Phylogenetic analysis demonstrated that scMYH(F1) and scMYH(F5) were monophyletic with fish adult fast-type MYHs, whereas scMYH(F2) and scMYH(F4) formed a cluster with fish slow-like fast-type MYH, and scMYH(F3) did with fish embryonic fast-type MYHs. Interestingly, juvenile silver carp predominantly expressed scMYH(F3) irrespective of acclimation temperatures at 10, 18 or 26 degrees C. The comparison among scMYH(F1), scMYH(F2) and scMYH(F3) in the deduced amino acid sequence revealed that the putative binding sites for ATP, actin, and essential and regulatory light chains in myosin subfragment-1 (S1) have high identities with each other (81-100%). However, their loop-1 and loop-2 regions in S1 were highly variable, suggesting their different functions. The deduced amino acid sequences of myosin subfragment-2 and L-meromyosin showed high identities of 90-91% and 86-90%, respectively, among the above three scMYHs.