We used a combination of high speed video and electrophysiological recordings to investigate the relationship between wing kinematics and the firing patterns of the first (b1) and second (b2) basalar muscles of tethered flying blowflies (Calliphora vicina). The b1 typically fires once during every wing stroke near the time of the dorsal stroke reversal. The b2 fires either intermittenly or in bursts that may be elicited by a visual turning stimulus. Sustained activation of the b1 at rates near wing beat frequency appears necessary for tonic maintenance of stroke amplitude. In addition, advances in the phase of b1 activation were correlated with both increased wing protraction during the down-stroke and increased stroke amplitude. Similar kinematic alterations was correlated with b2 spikes, and consequently, both muscles may function in the control of turns toward the contralateral side. The effects of the two muscles were evident within a single stroke period and decayed quickly. Kinematic changes correlated with b1 phase shifts were graded, suggesting a role in compensatory course stabilization. In contrast, b2 spikes were correlated with all-or-none changes in the wing stroke, a characteristic consistent with a role in mediating rapid turns towards or away from objects.