Pathogenic development in the corn smut fungus Ustilago maydis is controlled by a heterodimer of the two homeodomain proteins bE and bW which are encoded by the b mating type locus. The bE/bW heterodimer is thought to achieve its function as a transcriptional regulator of pathogenicity genes, either directly by binding to cis regulatory sequences or indirectly via a b-dependent regulatory cascade. In a screen for components of the b-dependent regulatory cascade we have isolated Rum1 (regulator U. maydis 1), a protein with similarities to the human retinoblastoma binding protein 2. Deletion of rum1 results in expression of several b regulated genes independently from their activation via the bE/bW heterodimer. rum1 mutant strains remain pathogenic, proliferate in planta, but fail to produce spores. The defect leads to an arrest in spore development at a defined stage before the spore wall is generated. Deduced from the highly conserved domain structure of Rum1 that includes a DNA-binding motif and a region known to facilitate the interaction with histone deacetylases, we propose that Rum1 functions as a transcriptional repressor through the modulation of chromatin structure.