Rainbow trout (family Salmonidae, Oncorhynchus mykiss) were acclimated to 5° and 20° C for 4 wk. The acclimatory response of the C-start (fast-escape swimming) and the molecular, biochemical, and contractile properties of fast-twitch muscle were investigated. With peptide mapping, no change was noted in the expression of myosin heavy chains in fast-twitch muscles with thermal acclimation. The myofibrillar ATPase activity of fast-twitch muscle isolated from individuals of each acclimation group was determined at 5° and 20° C The activity of myofibrillar ATPase was highly temperature dependent $(Q_{10} = 2.9)$. Acclimation temperature had no significant effect on ATPase activity measured at 5° or 20° C The thermal stability of the myofibrillar ATPase, however, was significantly increased following a period of acclimation to a higher temperature. Fast-twitch muscle contraction time (force onset to 50% relaxation) decreased from 32 ms to 12 ms with an acute change in temperature from 5° to 20° C $(Q_{10} = 1....