The numbers of sensory hairs of Drosophila melanogaster present an ideal model system to elucidate the genetic basis of morphological quantitative variation. Loci affecting bristle number can be identified and their properties studied by accumulating spontaneous mutations, by P element mutagenesis, by mapping factors causing divergence between selection lines and by the association of phenotype variation with molecular variation at candidate neurogenic loci. The consensus emerging from the application of all approaches is that much of the mutational and segregating variation affecting bristle number is attributable to alleles with large phenotype effects at a small number of candidate loci.