Work and heat production of white myotomal muscle fibres from dogfish were measured during sinusoidal movement (0.71-5.0 Hz) at 12 C. Stimulus phase (stimulus timing relative to movement) and duty cycle (stimulus duration as a fraction of movement cycle duration) were varied to determine the parameters optimal for power output and for efficiency (work/total energy output). Movements of 0.067 and 0.120L0 were used, where L0 is the muscle fibre length giving maximum force in an isometric tetanus. At each frequency of movement and duty cycle, the stimulus phase giving the highest power was the same as that giving the highest efficiency. In contrast, at each frequency and optimal stimulus phase, the dependence of power on duty cycle was very different from the dependence of efficiency on duty cycle. Power generally increased with increasing duty cycle, whereas efficiency decreased. Thus, high power can be achieved at the expense of efficiency by adjusting stimulus duty cycle. When stimulus phase and duty cycle were optimized, efficiency was always higher for the larger distance of movement. The efficiency of energy conversion can be maintained at a high level as the frequency of movement increases from 1.25 to 5.0 Hz.