The proneural genes of Drosophila are required for the segregation of neural precursors from the ectoderm. One of the proneural genes, achaete (ac), is responsible for the formation of a subset of larval and adult sense organs. We have examined the pattern of expression of ac and its regulation during embryogenesis. ac is expressed at particular locations of the ectoderm according to a highly dynamic and complex pattern. At each location the expression of ac occurs in two stages. First, ac is expressed in a cluster of cells ('competent' stage). This pattern of expression depends on an array of cis-regulatory sites which promote the expression of ac in clusters of cells at particular locations. Second, one cell of each cluster accumulates ac transcripts at a high level ('determined' stage) and becomes the neural progenitor cell. The restriction of the determined stage to one cell of the cluster requires the gene Notch. The transactivation of ac by sc (scute) or by other genes of the achaete-scute complex (AS-C) plays no role in defining the initial pattern of expression of ac, but might play a role in the shift from the competent to the determined level. We propose that the requirements of particular sense organs for different AS-C genes reflect the organization of this gene complex, rather than functional differences between the genes.