The neurogenic genes of Drosophila act during many different times and places during development. It is thought their role is to repress cell fate within a group of equivalent cells and thus allow the singling out of discrete numbers of precursors. Amongst the genes at the neurogenic locus, Enhancer of split is a family of seven related genes that encode proteins containing the basic helix-loop-helix motif characteristic of transcriptional regulators. Previous functional analyses of these genes have relied on deletions which eliminate many other genes. We have ectopically expressed two of the Enhancer of split basic helix-loop-helix genes, m5 and m8, to test their effect on the determination of the precursor cells of adult sensory organs. Ectopic expression of m5 or m8 before bristle precursor division results in loss of sensory bristles from all parts of the adult fly. Ectopic expression after bristle precursor division produces bristles with aberrant cuticular structures. We have also tested the effect of reducing Enhancer of split gene function using mitotic recombination and show that this de-represses the neural fate and produces supernumerary sensory bristle neurons. We conclude that the Enhancer of split basic helix-loop-helix genes inhibit neural fate during the selection of neural precursors, and that they also play a role in restricting the neuronal fate to one of the four progeny cells of the bristle precursor.