Cell-cell interactions play a fundamental role in the differentiation of nervous elements in constant patterns, both during embryogenesis and imaginal development. In this paper we analyse the role of genes of theachaete-scute andEnhancer of split complexes, plus the genesextramacrochaetae, Notch, Delta, andHairless in the patterning of sensory elements in the mesonotum ofDrosophila. The phenotypes of different alleles of these genes, including lethals in genetic mosaics, reveal their participation in two processes, the singling out from epidermal cells of sensory organ mother cells and their subsequent differentiation. Studies of allelic combinations of different genes lead to a model of the genetic interactions involved in the processes of pattern formation. In this model, theachaete-scute complex plays a central role, determining sensory organ mother cells and preventing neighbouring cells from following the same developmental pathway.