Recent results [Morata, G. & Kerridge, S. (1981) Nature (London) 290, 778-781] have shown that early Ultrabithorax- clones transform the posterior compartments of the adult meso- and metathoracic legs to prothorax. These transformations have not been seen in Ultrabithorax homozygous larvae, which are reported to show only transformations of the metathorax and the first abdominal segment to mesothorax [Lewis, E. B. (1978) Nature (London) 276, 565-570]. However, as the ventral surface of the larva does not exhibit sufficient markers to distinguish the posterior regions of these segments, cryptic larval transformations similar to those in the adult have been suggested (by Morata and Kerridge). We have further examined larvae of wild-type and various Ultrabithorax mutant genotypes, with particular attention to the dorsal surface. We find that Ultrabithorax homozygous larvae exhibit dorsal abnormalities consistent with transformations of the anterior metathorax and anterior first abdominal segment to mesothorax and of the posterior meso- and metathorax to prothorax as predicted by Morata and Kerridge; however, the posterior of the first abdominal segment remains untransformed. We suggest that in both larvae and adults the posterior first abdominal segment remains untransformed by Ultrabithorax mutations and that the unit of development with regard to the proximal bithorax complex consists of adjoining posterior and anterior compartments from neighboring segments rather than of segments themselves.