Live fibre bundles were isolated from the fast myotomal muscle of short-horned sculpin (Myoxocephalus scorpius L.) and isometric contractile properties and the force­velocity (P­V) relationship determined at 5, 10 and 15 °C. Experiments were carried out on winter- and summer-caught sculpins and on individuals acclimated for 6­8 weeks to either 5 or 15 °C (12 h:12 h light:dark). Maximum tetanic tension (P0) in fibres from 15 °C-acclimated fish increased from 125 kN m-2 at 5 °C to 282 kN m-2 at 15 °C (R10=2.3). For 5 °C-acclimated fish, P0 was 139 kN m-2 at 5 °C, but fell to 78 kN m-2 at 15 °C, consistent with a partial failure of excitation­contraction coupling at high temperatures. Peak force at 15 °C was increased 2.2 times following depolarisation with a high-K+ solution, but was unaffected by the addition of caffeine and/or eserine to the Ringer's solution. The results from winter- and summer-caught fish were similar to those from 5 °C- and 15 °C-acclimated sculpins respectively. In 15 °C-acclimated fish, the power output of muscle fibres calculated from the P­V relationship was 55 W kg-1 at 5 °C and 206 W kg-1 at 15 °C. The P­V relationship at 5 °C was significantly less curved in muscle fibres from 5 °C- than from 15 °C-acclimated fish. After normalizing the curves for P0 and Vmax, it was found that the change in curvature was sufficient to produce a 40 % increase in relative power output at 5 °C in cold-acclimated fish. The maximum contraction speed of muscle fibres at 15 °C was 2.4 times higher in 15 °C- than in 5 °C-acclimated fish. It was concluded that acclimation modifies the contractile properties of fast muscle fibres at both low and high temperatures.