These experiments were performed to determine how ectothermal animals maintain their locomotory ability during acute changes in muscle temperature, despite the large thermal dependence of the mechanical properties of their muscle. The electrical activity of the red (slow-twitch) and white (fast-twitch) muscle fibers of carp was monitored while the carp swam at various speeds at 10 and 20 degrees C. The patterns of recruitment of different fiber types were similar at both temperatures. At low speeds only the red muscle was active, whereas at high speeds the white muscle was active as well. The swimming velocity at which white muscle was initially recruited increased from 26 cm/s at 10 degrees C to 46 cm/s at 20 degrees C. These results suggest that the order of recruitment of motor units was the same at 10 and 20 degrees C but that the recruitment occurred over a narrower range of speeds at the low temperature. Hence, to generate the muscle power required to swim at a certain velocity, fish recruit more muscle fibers, which include faster fiber types when their muscle is cold than when their muscle is warm.