A DNA mediated transformation system has been developed for the filamentous fungus Cochliobolus lunatus. Transformants were obtained by using plasmid pAN 7–1 carrying the Escherichia coli hygromycin B phosphotransferase gene (hph) fused to an Aspergillus nidulans promoter. The integration of plasmid pAN 7–1 into the fungal genome altered the ability of this microorganism to transform progesterone.