In vivo stimulation of a relatively "silent" phasic crayfish motoneuron changes the ultrastructure of its synaptic terminals to a more tonic phenotype. The closer muscle of the crayfish claw is supplied by only 2 excitatory motoneurons, one of which is phasic and the other tonic. The ultrastructures of conditioned phasic, unconditioned phasic, and tonic motor terminals were compared. The terminals of the tonic motor axon were larger in cross-sectional area, had larger mitochondria, greater synaptic contact area, and were more varicose than unconditioned phasic terminals. Following long-term tonic stimulation of the phasic axon, its terminals became more varicose, mitochondrial cross-sectional area more than doubled, and synapses and mitochondria came into closer proximity, although mean terminal cross-sectional area did not change. Thus, the conditioned phasic terminals became more similar to those of the tonic motor axon. These changes in ultrastructure correlate with, and may be causally linked to, previously reported changes in neuromuscular synaptic physiology produced by in vivo tonic stimulation of this motoneuron. We conclude that the ongoing level of impulse activity can affect the ultrastructural differentiation of synaptic terminals and synapses of the phasic motoneuron.