Phosphate-water oxygen exchange was measured during ATPase activity of myosin subfragment 1, isolated from Lethocerus flight muscle. The result supports a mechanism including a rapid reversible ATP cleavage step followed by slow Pi release, so that extensive exchange occurs by multiple reversals of the cleavage step. Interaction with actin accelerates Pi release and reduces the extent of exchange. These properties are similar to vertebrate skeletal muscle subfragment 1. Differences in kinetic properties between insect flight and rabbit skeletal muscle exhibited in the fibres, particularly in respect of the strain activation of insect flight muscle, are not exhibited in the isolated myosin heads and are therefore probably due to organization within the fibre lattice.