A number of genetic loci, called neurogenic, have been found to be involved in directing the segregation of neural and epidermal lineages within the ectodermal germ layer ofDrosophila melanogaster. With the aim of understanding the regulation of this developmental function we have studied interactions of the lociN, DlandE(spl)among each other and with another locus(H), by means of increasing and decreasing the number of wild-type copies of one of these genes in the presence of mutations in another one. The results reveal functional community which exists among these neurogenic loci.E(spl)overlaps functionally with bothNandDlbecause genotypes involving only one copy ofE(spl)+and either anNorDlmutation are lethal. Furthermore the normalH+allele behaves as if it represses the activity of the 3 neurogenic loci; and, whereasE(spl)seems to be a close target ofHrepressing action, the influence ofHupon the other two seems to be indirect.