Calcium-activated insect flight muscle is further activated by small applied strain. We have investigated the mechanism of strain activation by measuring the extent of oxygen exchange between phosphate and water during ATP hydrolysis by Ca2+ -activated, chemically skinned fibers from the flight muscle of the giant waterbug Lethocerus indicus at different degrees of strain. The maximally activated insect fibres show a pattern of oxygen exchange which is well fitted by a single pathway of ATP hydrolysis. This differs from results with rabbit muscle, which show a more complicated form of oxygen exchange. We ascribe this difference to the different pattern of distribution of myosin heads on the thick filament. Calcium-activated, but unstrained, insect fibres also require more than a simple pathway of ATP hydrolysis to account for the pattern of oxygen exchange. The transition between the more complex pathway and the simple pathway occurs over a very narrow range of strain, centred at zero strain (rest length in the relaxed fibres). We describe how the experimental results of strain activation might occur, based upon Wray's description of filament geometries, but taking into account likely sarcomere mechanics. The pattern of oxygen exchange found in fully activated insect fibres suggests that the process of phosphate release from the actomyosin is rate limiting, implying that the main tension-generating state is an AM.ADP.Pi complex.