The molecular analysis of the Notch locus of Drosophila melanogaster demonstrated that it codes for a protein which shows homology to the epidermal growth factor as well as to the products of certain yeast genes involved in the control of the cell cycle (Wharton et al., 1985a; Breeden and Nasmyth, 1987). The structure of the protein suggests that Notch is involved in a cell interaction mechanism which controls the differentiation of several different tissues during development. Here we examine Notch expression during imaginal development using in situ hybridization to tissue sections and demonstrate that Notch is not expressed ubiquitously during the postembryonic stages, but rather is confined to specific tissues. During the larval and early pupal period Notch transcripts are predominantly localized in the imaginal discs and the central nervous system. In the middle and late pupal period the signal levels in these tissues drop dramatically and in the adult animal Notch transcripts are essentially detected only in the ovaries. In the larval stages the pattern of Notch expression appears to be closely correlated with mitotically active tissues, while in later stages this correlation appears less strict. The findings reported here indicate that there is a requirement for normal Notch function in a number of tissues at several developmental stages and that the pleiotropic phenotypic manifestation of Notch mutations is a context dependent developmental result. The observed association of Notch expression with mitotically active cell populations raises the possibility that Notch may play a role in the cell cycle.