We report the characterization of loss-of-function alleles of the homoeotic mutation Regulator of postbithorax (Rg-pbx) in Drosophila melanogaster. Rg-pbx is a dominant gain-of-function mutation which shows a transformation of posterior haltere to wing in the adult cuticle. This mutant phenotype mimics that of the bithorax complex lesion postbithorax (pbx). Loss-of-function alleles described here are lethal in the embryonic stage and affect the pattern of segmentation of the embryo. Examination of the terminal phenotype of null and hypomorphic alleles of Rg-pbx has shown that inactivation of the Rg-pbx gene leads to loss of the thoracic segments and the adjacent labial segment of the Drosophila embryo. An effect of the mutations is also seen in the seventh and eighth abdominal segments of embryos. The loss-of-function phenotype is similar to that described for the segmentation mutant hunchback (hb). Complementation tests show that Rg-pbx and hb are allelic. Temperature shift experiments using a temperature-sensitive loss-of-function allele show that the Rg-pbx gene product is required early in embryogenesis. We further report that the dominant Rg-pbx phenotype is sensitive to the gene dosage of another segmentation-controlling gene, fushi tarazu (ftz). Flies carrying a mutant copy of the ftz gene in trans to Rg-pbx show a dramatic enhancement of the penetrance of the homoeotic mutant phenotype. We were also able to demonstrate a suppression of the Rg-pbx phenotype by the addition of a duplication for the ftz+ gene to an Rg-pbx stock. Examination of the phenotype of ftz Rg-pbx- double-mutant embryos did not reveal a clear pattern of epistasis between the genes nor was absolute additivity of phenotype seen. A possible formal relationship between Rg-pbx, ftz, and the postbithorax (pbx) locus is proposed.