In short-horn sculpin Myoxocephalus scorpius, the power requirements for fast-start swimming and the length-specific velocity of the curvature wave travelling down the spine (U) were not influenced significantly by acclimation to summer and winter conditions at test temperatures of 5 and 15 o C. However,in-vivo and in-vitro muscle performance exhibited acclimation responses at 15 o C. Seasonal acclimation altered the escape performance curves for power and U significantly over a wider temperature range of 0.8-20 o C. U was significantly higher at 20 o C in the summer- than winter-acclimation group. The acclimation of lower levels of physiological organization at 15 o C may thus serve to extend the thermal limits for escape performance in summer acclimated fish.