Separate phasic or tonic nerves allotransplanted to reinnervate a denervated slow superficial flexor muscle (SFM) in the abdomen of adult crayfish regenerate synaptic nerve terminals with phasic or tonic properties. To test competitive interactions between tonic and phasic axons, we allotransplanted the sixth abdominal ganglion with its third nerve root containing a mixture of phasic and tonic axons onto the denervated SFM. The resulting reinnervation of the SFM was compared to the normal innervation on the contralateral intact SFM, which receives innervation only from tonic motoneurons. Variable sizes of excitatory postsynaptic potentials indicated that 2-3 axons innervated each muscle fiber of the SFM in both the allotransplant and normal preparations. Compared to the normal tonic terminals on the intact contralateral side, the allotransplanted synaptic terminals had more phasic-like properties; specifically, they gave rise to larger synaptic potentials, had a lower mitochondrial content and contained a higher density of active zone dense bars per synapse. Moreover, prolific sprouting of the axons in the regenerated nerve, typical of phasic axons, points to more vigorous regeneration of phasic rather than tonic axons to the denervated SFM. In keeping with this prolific axon sprouting, there was both a much higher density of innervation in the allotransplanted SFM compared to the normal SFM, and a higher frequency of extrasynaptic active zones in regenerated terminals of the mixed nerve compared to those of the tonic nerve. Thus, an allotransplanted mixed nerve regenerates mainly phasic axons and synapses on the slow denervated SFM, demonstrating the instructive nature of the neuron in synapse specification, as well as the permissive nature of the target muscle.