1. We measured the influence of temperature on maximum velocity of shortening (Vmax) of red muscle in carp in order to better understand the influence of temperature on locomotory performance. 2. A stable red muscle bundle preparation containing about 100 muscle fibres was developed. The bundles could not be activated directly by electrical stimulation, but rather contained sufficient nervous tissue so that acetylcholine released from the nerve terminals caused activation of the muscle. A high level of activation was achieved (116 kN/m2) by adding a combination of a 1 mM-caffeine and 10(-5) g/ml eserine to physiological Ringer solution and electrically stimulating the preparation. 3. Force-velocity characteristics were determined at 10 and 20 degrees C by the force clamp method. The data were well fitted by a hyperbola not constrained to pass through P0 = 1 (where P0 is the isometric force). The mean Vmax at 10 degrees C was 3.55 +/- 0.26 muscle lengths/s (ML/s) (n = 6) and at 20 degrees C, 5.71 +/- 0.29 ML/s (n = 6). The mean Q10 for Vmax was 1.63 +/- 0.07 (n = 6). The a/P0* (Hill constant) and Po* (where P0* is the extrapolated load at zero velocity) were 0.49 +/- 0.06 (n = 6) and 1.19 +/- 0.04 (n = 6) respectively at 10 degrees C and 0.29 +/- 0.06 (n = 6) and 1.51 +/- 0.20 (n = 6) respectively at 20 degrees C. 4. The mean Q10 for maximum isometric tension was 1.13 +/- 0.02 (n = 6). The maximal power generation was 59.7 +/- 2.3 W/kg (n = 6) at 10 degrees C and 94.3 +/- 3.2 W/kg (n = 6) at 20 degrees C representing a Q10 of 1.58. The Q10 is less than the product of Q10s for P0 and Vmax because of the greater curvature of the force-velocity curve at 20 degrees C. 5. The 1.63-fold higher Vmax at 20 degrees C than at 10 degrees C enables fish to swim with a 1.6-fold faster muscle shortening velocity, V, at the higher temperature. Thus at both 10 and 20 degrees C, red muscle is used only over the same narrow range of V/Vmax (0.18-0.36), where isolated muscle experiments suggest that power and efficiency are maximal. Thus V/Vmax appears to be an effective design constraint which limits the range of velocities over which muscle is used in vivo at different temperatures.