Analyses of Delta (Dl) temperature-sensitive mutants reveal that reductions in Dl function during metamorphosis of Drosophila melanogaster can lead to multiplication or loss of bristle organs on the adult notum, depending on the developmental interval during which such mutants are pulsed at restrictive temperature. Site-dependent macrochaeta multiplication or loss results from pulses 0-21 hr after puparium formation (APF). Microchaeta multiplication results from pulses 7-30 hr APF, while microchaeta loss results from pulses 24-47 hr APF. Supernumerary bristle organs that develop in Dl mutants consist of the normal complement of trichogen, tormogen, neuron, and thecogen cells, and appear to arise due to the specification of super-numerary bristle first-order precursor (pI) cells within proneural groups in the developing notum. Loss of complete bristle organs in Dl mutants is correlated with the differentiation of all four progeny of the pI cell into neurons. We conclude that Dl function is required for cellular interactions central to the correct specification of pI cells within proneural groups, as well as for morphogenesis of the bristle organ itself.