The interaction of three neurogenic loci viz. Delta, Enhancer of split and Notch, and a related gene, Hairless, of Drosophila melanogaster was investigated at the adult morphology level by measuring the effects of the mutations of the three other genes on the expression of the recessive lethal antimorphic Abruptex mutations of the Notch locus. The Abruptex mutations were also coupled in cis or trans with facet-glossy or split mutations of the Notch locus. In some of the experiments, the genotype of the fly was homozygous for either facet-glossy or split mutation or their wild type alleles but heterozygous for the Abruptex. Facet-glossy is located in a large intron of the locus, whereas split is located in the same exon as Abruptex. In all compounds studied, Delta suppressed the expression of Abruptex while Hairless and Enhancer of split enhanced it. The interactions of the four genes studied were allele specific, suggesting an interaction at the protein level. The comparison of the results presented in this study on the interaction of the neurogenic genes with other results on the same subject suggests that the interactions are similar in embryonic and imaginal development.