Genes within subdivision 1B of the X-chromosome ofDrosophila melanogasterare known to affect the development of both the central (CNS) and the peripheral (PNS) embryonic nervous system. In this paper we describe the phenotypes of embryos hemizygous for terminal and interstitial deletions of region 1B1-1B10, and of embryos carrying different mutations in certain genes of this region. A minimum of 6 genetic functions that are involved in neural development can be defined in this region. Three of these genes, mapping to thel'sc(a gene of the achaete-scute complex, ASC),elavandvndloci, affect major and apparently different aspects of CNS development. Two additional genes of the ASC,acandscα, also play a role in CNS development, although their participation can only be demonstrated under certain conditions. Finally, in the rightmost part of the region uncovered by the deletionDf(1)260.1, two not yet well separated functions are found to be required for embryonic CNS and compound eye development, respectively. Of these two functions, the embryonic one is similar to the ones ofacandscα, and can therefore be considered as a new element of the ASC.