We have isolated a novel class of gain-of-function mutations at the Bearded (Brd) locus which specifically affect the development of adult sensory organs in Drosophila. These Brd alleles cause bristle multiplication and bristle loss phenotypes resembling those described for the neurogenic genes Notch (N) and Delta (Dl). We have found that supernumerary sensory organ precursor (SOP) cells develop in the proneural clusters of Brd mutant imaginal discs; like normal SOPs, these are dependent on the function of the proneural genes achaete and scute, and express elevated levels of ac protein. At cuticular positions exhibiting the Brd bristle loss phenotype, we have found that the progeny of the multiplied SOPs develop aberrantly, in that neurons and thecogen (sheath) cells appear but not trichogen (shaft) and tormogen (socket) cells. This appears to represent a transformation of the pIIa secondary precursor cell within the SOP lineage to a pIIb secondary precursor cell fate. These results suggest that Brd gain-of-function alleles interfere with Notch pathway-dependent cell-cell interactions at two distinct stages of adult sensory organ development. We have also identified enhancers and suppressors of the Brd dominant phenotypes; these include both previously characterized mutations and alleles of apparently novel loci. Finally, we have found that Brd null mutants are viable and exhibit no mutant phenotypes, suggesting that Brd may be a component of an overlapping function.