We have investigated the effects of mutations of the pleiohomeotic (pho) locus (formerly called l(4)29) on embryonic and adult development of Drosophila melanogaster. The normal function of pho is vital for pattern formation during embryonic and adult development. The hypomorphic, adult-viable phocv allele produces maternal-effect embryonic lethality, and the lethal embryos show homeotic transformations of head, thoracic, and abdominal segments and abnormal development of the central and peripheral nervous systems. Hypomorphic and amorphic pho alleles are recessive lethals with the lethal individuals showing partial homeotic transformations of antennae, legs, abdominal segments, and internal structures. In addition, pho mutations product pattern abnormalities in the legs and a novel sixth tarsal segment phenotype. The pho adult phenotypic effects are restricted to discrete spatial regions of the imaginal discs. In leg discs these effects are localized in the upper medial quarter of the discs and show a striking correlation with the organization of positional information as proposed by the polar coordinate model.