The neurons of the sensory receptors on the wing of Drosophila melanogaster have highly characteristic axon projections in the central nervous system (CNS). The morphology of these projections was studied in flies bearing mutations that affect cell surface molecules thought to be important in axon guidance. The animals used were mutant for the fasciclinI (fasI), fasciclinII (fasII), fasciclinIII (fasIII) and neurally altered carbohydrate (nac) genes. Axon populations were visualized by staining with DiI and light-reacting the dye with diaminobenzidine to yield permanent preparations. The fasI, fasII and fasIII mutants as well as the nac mutant display altered axonal trajectories in the CNS. One phenotype seen in fasII mutants and in animals mutant for both fasI and fasIII was extra branching within the axon projection pattern. A second phenotype observed was a reduction or complete loss of one of the tracts, apparently due to the axons shifting to a neighboring tract. This was seen in the most extreme form in nac mutants and to a lesser degree in fasIII mutants. To determine if the mutations discussed here affected axon guidance, wing discs were analyzed using the antibody 22C10 to label sensory neurons in the wing during metamorphosis. Both misrouting of axons and the appearance of ectopic neurons in the wing were observed. In the fasI:fasIII , the fasII and the nac mutants, there was misrouting of sensory axons in the developing wing. In addition, the fasII and nac mutants displayed ectopic sensory neurons in the wing. This implies that the cell surface molecules missing (fasciclins) or modified (by the nac gene product), in these mutants may play a role in both neurogenesis and axon guidance.