The Notch locus of Drosophila plays an important role in cell fate decisions within the neurogenic ectoderm, a role thought to involve interactions at the cell surface. We have assayed the requirement for Notch gene expression in epidermal cells by two kinds of genetic mosaics. First, with gynandromorphs, we removed the wild-type gene long before the critical developmental events to produce large mutant clones. The genotype of cells in large clones was scored by means of an antibody to the Notch protein. Second, using mitotic recombination, we removed the gene at successively later times after completion of the mitotically active early cleavage stages, to produce small clones. These clones were detected by means of a linked mutation of cuticle pattern, armadillo. The results of both experiments demonstrate a requirement for Notch expression by epidermal cells, and thus argue against the model that the Notch product acts as a signal required only in the neuroblast to influence neighboring epidermal cells. The mitotic recombination experiment revealed that Notch product is required by epidermal cells subsequent to neuroblast delamination. This result implies that the Notch gene functions to maintain the determined state of epidermal cells, possibly by mediating cell surface interactions within the epidermis.