The pleiohomeotic (pho) gene is a member of the Polycomb (Pc) group genes, whose common role in development is to regulate the spatial expression of the ANT-C and BX-C genes. The goal of this research was to characterize the pleiotropic effects of mutant alleles of pleiohomeotic on Drosophila development in a series of genetic and developmental analysis. The pho maternal effect phenotypes include abnormal head involution, partial homeotic transformation, and a variety of segmental defects occurring predominantly in the second thoracic segment and in odd number of abdominal segments. Adults homozygous for pho mutant alleles have several homeotic transformation phenotypes, including transformation of the antenna, leg, and abdominal segments. pho alleles show a synergistic interaction with mutant alleles of other Pc group genes, and this interaction is sensitive to changes of dosage of Pc group genes. Unlike other Pc group genes, pho does not dramatically interact with the ANT-C and BX-C genetically and molecularly. Surprisingly, pho[superscript] cv maternal effect alters several early developmental events, which may be a consequence of its causing abnormal expression of ftz. SEM analysis, analysis of embryonic denticle patterns, and the alterations in the patterns of en expression suggest that pho[superscript] cv may affect genes that function even earlier in development than ftz. We conclude that pho has a much greater role in development than simply regulating the expression of the ANT-C and BX-C homeotic genes. A model that may explain the complex phenotypes of the Pc group mutations is that the pho gene product forms a metameric complex with the products of other Pc group genes and that this complex regulates the action of homeotic genes. Mutations of any of the Pc group genes whose product is in the complex may alter the equilibrium level of the complex, and may alter the functional specificity of the complex.