A basidiomycete phytopathogenic fungus, Ustilago violacea, was transformed with pUCH1, a bacterial plasmid containing the hygromycin (Hyg)-resistance (R) hygB gene fused to a promoter from the ascomycete Cochliobolus heterostrophus. After lithium acetate/polyethylene glycol treatment of whole sporidial cells, U. violacea transformants appeared on Hyg-agar at a frequency of 60-80 per μg pUCH1 DNA. The HygR phenotype was 100% stable in these transformants for at least 30 generations of mitotic growth under non-selective conditions. Southern DNA-DNA hybridization revealed multiple integrations of the pUCHI plasmid into the U. violacea nuclear DNA. In addition, Escherichia coli transformants appeared at a frequency of 12 per Jiμg nuclear fraction DNA from HygR U. violacea transformants; these E. coli consistently contained a deleted pUCHI plasmid. This latter result suggested the low-frequency production of circular molecules by recombination within the integrated sequences. © 1989.