In this paper we describe the spatiotemporal pattern of sensillum progenitors (SOPs), as well as the way in which these cells segregate from the ectoderm and proliferate. The birthdate of SOPs was determined by applying short heat pulses to embryos carrying the Nintra construct [Struhl et al. (1993), Cell, 74:331-345] which allows the overexpression of the active Notch protein at defined developmental stages and thereby eliminates SOPs which would normally segregate during these stages. Our results show that sensillum progenitors appear in several waves which to some degree respect sensillum modality, as well as dorsoventral sensillum location. The four early SOPs (stage 10) give rise exclusively to multiply innervated sensilla, chordotonal organs, and some multidendritic neurons. The second wave (early stage 11) produces the remaining chordotonal organs, some multidendritic neurons, and the dorsal singly innervated mechanosensilla. The third wave (late stage 11), in a dorsal-to-ventral succession, gives rise to the lateral and ventral singly innervated hair and papilla sensilla. Labeling developing SOPs with specific markers demonstrates that only progenitors of subepidermally located chordotonal organs and multidendritic neurons delaminate, whereas progenitors of external sensilla are born and proliferate within the ectodermal layer.