The sharp, nonoverlapping boundaries of the major and minor receptive fields of the mechanosensory neuron Pv of the leech, as well as the mutual exclusion during embryonic development of growing axon branches belonging to the same Pv cell, have suggested that peripheral axon arborization of these neurons is constrained by a process of neuronal self-avoidance. To provide a direct experimental test of this proposal, the development of the major and minor receptive fields of the Pv neuron was studied in embryos of the leech, Haementeria ghilianii, after surgically preventing or delaying the outgrowth of the axon branches which establish only a minor or only the major field of that neuron. As predicted by the proposal of self-avoidance, interference with the outgrowth of a minor field axon branch resulted in the spread of the major field axon branch into what is normally minor field territory. Conversely, similar interference with the establishment of the major field resulted in the spread of the minor field axon branches into what is normally major field territory. The findings presented here indicate that neuronal self-avoidance does play a significant role in the development of mechanosensory receptive field structure but suggest also that the detailed pattern of arborization of the sensory axons is guided by prespecified pathways of only ephemeral availability or recognizability.