A newly identified gene is described that is required for the maintenance of normal identities in many of the body segments of the fly. The effects of mutants in this gene, which is called Polycomblike (Pcl), suggest that its wild-type allele functions in the regulation of the bithorax gene complex (BX-C) and the Antennapedia gene complex (ANT-C). Evidence in favor of this idea derives from (1) the close correspondence between segmental transformations caused by Pcl mutants and those caused by dominant gain-of-function mutants in the BX-C and ANT-C, (2) the interactions observed between Pcl mutants and mutants in these complexes, and (3) the dependence upon BX-C and ANT-C dosage of the severity of at least one of the transformations caused by Pcl mutants. Arguments are presented that the control of the BX-C and ANT-C by Pcl+ is negative in nature. The results of clonal analysis experiments indicate that, at least for the BX-C, Pcl+ exerts this control until late in development. Since the wild-type allele of another gene, called Polycomb (Pc), has previously been shown to have many of the same properties as Pcl+, it appears that the BX-C and perhaps also the ANT-C are continuously regulated during development by at least two and probably several other genes.