The Polycomb group genes in Drosophila are involved in the stable and inheritable repression of gene expression. The Polycomb group proteins probably operate as multimeric complexes that bind to chromatin. To investigate molecular mechanisms of stable repression of gene activity in vertebrates we have begun to study Xenopus homologs of Polycomb group genes. We identified the Xenopus homologs of the Drosophila Polycomb gene and the bmi-1 gene. bmi-1 is a proto-oncogene which has sequence homology with the Polycomb group gene Posterior Sex Combs. We show that the XPolycomb and Xbmi-1 genes are expressed in overlapping patterns in the central nervous system of Xenopus embryos. However, XPolycomb is also expressed in the somites, whereas Xbmi-1 is not. We further demonstrate that the XPolycomb and Xbmi-1 proteins are able to interact with each other via conserved sequence motifs. These data suggest that also vertebrate Polycomb group proteins form multimeric complexes.