The gene poxn codes for a transcriptional regulator that specifies poly-innervated (chemosensory), as opposed to mono-innervated (mechanosensory), organs in Drosophila. The ectopic expression of poxn during metamorphosis results in a transformation of the morphology and central projection of adult mechanosensory organs toward those of chemosensory organs. Here we show, by electron microscopy analysis of normal and transformed bristles and by Dil labeling of the innervating neurons, that poxn also controls the number of neurons. To determine whether poxn can transform not only the sense organ precursor cells but also their daughter cells, we examine the effects of the ectopic expression of poxn at different stages of the lineage, and we conclude that poxn can act at a late stage to affect the fate of the undifferentiated neuron.