Turning movements in fixed flight in the milkweed bug, Oncopeltus, are brought about by asymmetrical alteration of the wing amplitudes of the two sides. Extracellular recordings were obtained from the direct non-fibrillar coxo-subalar muscle during directional changes in flight. The muscle is differentially activated on the two sides, such that during a right turn the frequency of nerve impulses increased in the right muscle and decreased in the contralateral muscle. The action of the coxo-subalar muscle on the wing stroke is discussed.