1. The efficiency of the metathoracic tergosternal muscle of the locust Schistocerca americana was examined by simultaneously measuring work output from the muscle and oxygen consumption by the muscle. The work output was determined using the work-loop technique in which the muscle is subjected to periodic strain and to phasic stimulation in the strain cycle. The area of the loop formed by plotting muscle force against muscle length over a cycle is the net work output for that cycle. 2. The tergosternal muscle is a synchronous, parallel-fibred muscle containing two motor units with similar contraction kinetics. The average twitch rise time (30 degrees C) was 15 ms, the twitch duration (to 50% relaxation) was 26 ms, and the peak twitch tension with both units active was 73 kN m-2. The maximum mechanical power output during sinusoidal shortening at 20 Hz with both motor units active and stimulated once per cycle averaged 37 W kg-1. 3. The overall efficiency of the tergosternal muscle averaged 6.4% (range 4-10%) where efficiency is defined as the ratio of the net work done (20 Hz sinusoidal strain, 1 stimulus per cycle, optimum strain amplitude and stimulus phase) to the caloric equivalent of the oxygen consumed. The efficiency was independent of the duration of the test period (examined range = 10-30 s) and the same when both motor units were active as when only one was stimulated. 4. Stimulating the muscle with two stimuli per cycle (interstimulus interval = 6 ms) increased the work per cycle by about 13% above that with single stimuli per cycle, but the muscle fatigued more rapidly and after 15-25 s the power output was less with two stimuli per cycle than with one. The efficiency with two stimuli per cycle was slightly less than that with one shock per cycle. 5. The oxygen consumption during normal work cycles at 20 Hz with optimum stimulus phase and strain was greater by about 15% than the oxygen consumption during isometric contractions at the same frequency.