1. Motor units in the thin transversus abdominis muscle of the garter snake were identified and physiologically characterized in the living state. Motor unit fibres, and fibres chosen randomly to serve as controls, were subsequently excised and subjected to biochemical analyses. 2. The metabolic capacity of fibres was assessed by measuring activities of three enzymes, each representing a different metabolic pathway. The microchemical enzyme assays were performed using enzyme extraction preparations of whole single fibres. 3. Metabolic capacity ranged widely among the muscle's entire fibre population, even among fibres of the same type. In contrast, enzyme activities of twitch fibres belonging to individual motor units were, within analytical error, identical. 4. Twitch contraction times of individual fibres within one motor unit were similar, compared to a wide range of contraction times observed among fibres of the same type but belonging to different motor units. 5. When several motor units were studied in one muscle, a systematic relationship was observed among motor unit tension, enzymatic profile and contraction time. As motor unit tension increased, fibres exhibited greater capacities for glycolytic and high-energy phosphate metabolism, diminished capacity for oxidative metabolism, and faster twitch contraction times. 6. Given the great diversity of metabolic and contractile properties exhibited within the fibre population, the uniformity of such properties within motor units indicates that neural influence dominates over other extrinsic factors present in the microenvironment of the muscle fibres.