The ultrastructural differentiation and central projection of identified bristle mechanosensory neurons were examined in Drosophila mutants lacking action potentials. Two mutations, parats1 and napts, are known to block axonal conduction in centrally located neurons at high temperatures. Their effects on epithelial sensory cells, which are derived from imaginal disks during pupation, have not been determined. Furthermore, the parats1 napts double-mutant flies are lethal at all temperatures; thus the synergistic effect of these mutations on neurons has not yet been studied. It is possible to examine the above questions in genetic mosaics. By monitoring a reflex response involving identified bristle sensory cells, we found that the 2 mutations exert similar effects on these epithelial sensory cells as seen in central neurons. This also indicates that the action potential mechanisms in both epithelial sensory cells and central neurons are under similar genetic control. The parats1 napts double-mutant sensory cells in mosaics are nonfunctional at all temperatures, providing an opportunity to examine, at the single cell level, the development of neurons with activity block. Ultrastructural specializations typical of epithelial sensory cells were found in the double-mutant cells. Cobalt backfilling experiments showed that central projections of these nonfunctional sensory cells were not altered, as compared with the active contralateral sensory cells. Therefore, blockage of the action potential mechanism in individual sensory cells has no effect on their pathfinding and arborization.