| Summary: | Bundles of 3-10 live fast fibres were isolated from the abdominal myotomes of cod ( Gadus morhua L.) 13-67 cm in length. The preparations performed work under conditions simulating their activity during swimming: sinusoidal length changes were imposed about in situ fibre length, and the fibres were stimulated at a selected phase in each cycle. Strain amplitude, and the number and timing of stimuli were chosen to give maximum power output over a wide range of cycle/tailbeat frequencies. For each preparation power output was maximal at a particular frequency, although the peaks were rather broad. As the size of the fish increased the cycle frequency for maximum power output (f opt ) decreased, from 12.5 Hz (13 cm fish) to 5 Hz (67 cm fish) (f opt = 1.67 L -0.52, where L is body length).
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