Polycomb group (PcG) proteins are conserved transcriptional repressors that cooperate to stably maintain the silenced state of several developmental regulator genes, in cells in which these genes should not be expressed. Thus-far characterized PcG proteins have the ability to interact with and/or modify chromatin. We have found that the previously uncharacterized Drosophila PcG gene super sex combs (sxc) encodes Ogt, the highly conserved glycosyltransferase that catalyzes the addition of N-acetylglucosamine (GlcNAc) sugar residues to proteins in the nucleus and cytosol. GlcNAc-modified proteins can be detected on Drosophila chromatin at sites of PcG protein binding. Furthermore, the PcG protein Polyhomeotic (Ph) is glycosylated by Sxc/Ogt in vivo. sxc/Ogt-null mutants lack nuclear GlcNAcylation, and fail to maintain Polycomb transcriptional repression even though PcG protein complexes are still bound to the chromatin of their target genes. Polycomb repression appears to be a critical function of Sxc/Ogt in Drosophila and may be mediated by the glycosylation of Polyhomeotic.