The pump responsible for gill ventilation in the crayfish is operated by 11 muscles of the second maxillae. These muscles arc described and named and their action analyzed by the recording of nerve and muscle potentials. During quiet breathing, the normal motor command to each muscle is a short, high-frequency burst of evenly spaced action potentials (7–25 spikes at average frequencies of 60–140/sec). Histological and physiological evidence shows that the muscles represent mixed populations of fiber types. Most fibers are of the "gradedly responding" phasic type. Although methylene blue staining reveals two axons to each muscle, the normal motor command arrives via one axon only. The second may be a "fast" axon operating only in exceptional circumstances. No evidence has been found of a peripheral inhibitory system.