The gene pox-neuro (poxn), which encodes a possible transcriptional regulator including a paired domain, specifies the differences between monoinnervated and polyinnervated sensory organs in the embryo. A detailed analysis of this gene, and in particular, an analysis of its function in the adult sensory organs, has so far been hampered by the unavailability of loss-of-function mutations. Here, we report the isolation of loss-of-function mutations of poxn and show that the chemosensory bristles are transformed into mechanosensory bristles in mutant flies. The external morphology of putative chemosensory bristles, number of innervating neurons, and cell division pattern are all affected in the mutants, showing that poxn is strictly required for development of the adult chemosensory bristles. In addition, the formation of some precursor cells is suppressed in the mutants, suggesting that poxn is also required for formation of the precursors of chemosensory bristles.