Bundles of myofibrils prepared from the dorsal longitudinal flight muscles of giant water bugs show oscillatory contractile activity in solutions of low ionic strength containing ATP and 10-8-10-7 M Ca2+. This is due to delay between changes of length and changes of tension under activating conditions. The peculiarities of insect fibrillar muscle which give rise to this behavior are (1) the high elasticity of relaxed myofibrils, (2) a smaller degree of Ca2+ activation of ATPase activity in unstretched myofibrils and extracted actomyosin, and (3) a direct effect of stretch on ATPase activity. It is shown that the cross-bridges of striated muscle are probably formed from the heads of three myosin molecules and that in insect fibrillar muscle the cycles of mechanochemical energy conversion in the cross-bridges can be synchronized by imposed changes of length. This material is more suitable than vertebrate striated muscle for a study of the nature of the elementary contractile process.