A motor unit in the stomach of the blue crab, Callinectes sapidus, consists of four separate muscles involved in different aspects of the trituration and filtering of food. Motor nerve terminals to two of the muscles (CPV7a and GM5) release small amounts of transmitter (low-output) while those to the other two muscles (CV2 and CV3) release between three and five-fold greater amounts (high-output). Structural features underlying the disparity in synaptic strength were analysed with thin serial-section electron microscopy. Nerve terminals were similar in their volume percent of mitochondria, clear vesicles and dense core vesicles among the four muscles. This was also the case for the number and size of synaptic contacts. However, presynaptic dense bars representing active zones were longer and occurred more frequently at high-output synapses than at low-output ones. High-output synapses were also characterized by the close spacing of adjacent dense bars. The longer and more closely spaced dense bars at high-output synapses would be factors in the generation of larger synaptic potentials in these terminals compared to their low-output counterparts. Other factors, however, need to be considered to fully account for the physiological differences in synaptic strength among the four muscles.