Hairy-related proteins include the Drosophila Hairy and Enhancer of Split proteins and mammalian Hes proteins. These proteins are basic helix-loop-helix (bHLH) transcriptional repressors that control cell fate decisions such as neurogenesis or myogenesis in both Drosophila melanogaster and mammals. Hairy-related proteins are site-specific DNA-binding proteins defined by the presence of both a repressor-specific bHLH DNA binding domain and a carboxyl-terminal WRPW (Trp-Arg-Pro-Trp) motif. These proteins act as repressors by binding to DNA sites in target gene promoters and not by interfering with activator proteins, indicating that these proteins are active repressors which should therefore have specific repression domains. Here we show the WRPW motif to be a functional transcriptional repression domain sufficient to confer active repression to Hairy-related proteins or a heterologous DNA-binding protein, Ga14. This motif was previously shown to be necessary for interactions with Groucho, a genetically defined corepressor for Drosophila Hairy-related proteins. Here we show that the WRPW motif is sufficient to recruit Groucho or the TLE mammalian homologs to target gene promoters. We also show that Groucho and TLE proteins actively repress transcription when directly bound to a target gene promoter and identify a novel, highly conserved transcriptional repression domain in these proteins. These results directly demonstrate that Groucho family proteins are active transcriptional corepressors for Hairy-related proteins and are recruited by the 4-amino acid protein-protein interaction domain, WRPW.