In the corn smut fungus Ustilago maydis, pathogenic development is controlled by the b mating type locus that encodes the two homeodomain proteins bE and bW. A heterodimer of bE and bW controls a large set of genes, either directly by binding to cis regulatory sequences or indirectly via a b-dependent regulatory cascade. It is thought that several of the b-regulated genes contribute to processes involved in pathogenicity. In a screen for components of the b-dependent regulatory cascade we have isolated Hda1, a protein with homology to histone deacetylases of the RPD3 class. Hda1 can substitute for the histone deacetylase RPD3 in Saccharomyces cerevisiae, showing that it functions as a histone deacetylase. Deletion of hda1 results in the expression of several genes that are normally expressed only in the dikaryon, among these are several genes that are now expressed independently from their activation by the bE/bW heterodimer. hda1 mutant strains are capable to infect corn, and the proliferation of dikaryotic hyphae within the plant appears comparable to wild-type strains during initial developmental stages. Upon karyogamy, however, the proliferation to mature teliospores is blocked. The block in sporogenesis in Deltahda1 strains is probably a result of the deregulation of a specific set of genes whose temporal or spatial expression prevent the proper developmental progress.