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 of Drosophila melanogaster. With the aim of understanding the regulation of this developmental function we have studied interactions of the loci N, Dl and E(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 both N and Dl because genotypes involving only one copy of E(spl)+ and either an N or Dl mutation are lethal. Furthermore the normal H+ allele behaves as if it represses the activity of the 3 neurogenic loci; and, whereas E(spl) seems to be a close target of H repressing action, the influence of H upon the other two seems to be indirect.