The phytopathogenic fungus Ustilago maydis exists in a yeast-like haploid form and as a filamentous dikaryon. Only the dikaryon can infect corn plants. We have isolated a gene, egl1, that is not expressed in haploid cells but strongly induced in the filament. Molecular and biochemical analyses revealed that egl1 encodes a cellulase. By immunogold labelling, secreted protein could be detected at the hyphal tip. Mutants deleted for egl1 are viable and are affected neither in filament formation nor in pathogenic development under the conditions tested.