The earliest events in the development of the central nervous system in insect embryos involve the differentiation of a stereotyped pattern of individually identified neuronal precursor cells, called neuroblasts (NBs), each of which generates a stereotyped family of neuronal progeny. After gastrulation, the midventral region of the ectoderm becomes a neuroepithelium; it is from this sheet of seemingly uniform ectodermal cells that certain cells enlarge to become NBs while the rest of the cells either die or acquire other nonneuronal phenotypes. Here we focus on three aspects of neurogenesis. First, we examine the morphological changes associated with the differentiation of neuronal precursor cells and nonneuronal support cells. The differentiation of each cell type is reflected by its morphology; moreover, the identity of individual cells of certain cell types (e.g., a particular NB) is reflected by their position within the neuroepithelium. Second, we show that there is a characteristic temporal sequence of differentiation in the stereotyped pattern of NBs. Third, we show that this stereotyped pattern of NBs varies in a segment-specific way by the addition or deletion of particular neuronal precursor cells. These studies on the events of early neurogenesis set the stage for the experimental manipulations described in the following paper (C. Q. Doe and C.S. Goodman, 1985, Dev. Biol. 110, 206-219).