A high frequency of homeotic transformation appears in Df(3)red/+ progeny of Df(1)snC128/+ females. Generally, the metathoracic appendages are partially transformed into mesothoracic ones. Df(1)snC128 includes a small region of the X chromosome: 7D1 to 7D5-6. Hypodosage of this region is mainly effective at the level of the maternal genotype, and the effect is probably due to hypodosage of the wild-type allele of the gene fs(1)h. Df(3)red has an effect that is mainly, if not exclusively, zygotic, probably due to hypodosage of the wild-type allele of Rg-bx. The frequencies of transformed flies resulting from the interaction between Df(1)snC128 and Df(3)red are not very sensitive to external conditions and genetic background. Studies of the interactions between Df(1)snC128 and other mutations or deficiencies of chromosome 3 [Rg-pbx, bx, pbx, Ubx1, Ubx130, Ubx80, Df(3)P9] reveal an analogy between the hypodosage effect of region 7D1-7D5-6 and the effects of ether treatment of blastoderm stage eggs. The role of the gene fs(1)h in the process of segment determination is discussed in the light of these results.