Coniothyrium minitans was successfully co-transformed with the uidA (β-glucuronidase) and the hygromycin-resistance (hph) genes. Both were under the control of the glyceraldehyde-3-phosphate promoter from Aspergillus nidulans. Hygromycin resistance was used as a selectable marker for transformation. In successive transformation experiments, transformation frequencies of up to 1000 transformants μg−1 of plasmid DNA were obtained for isolate A69. Of the ten monospore hygromycin-resistant cultures tested, nine also expressed the uidA gene. Expression of hph and uidA was stable in all transformants after several months of successive subculturing on non-selective medium, and after passage through a sclerotium of Sclerotinia sclerotiorum. Southern hybridization analyses showed all transformants carried multiple copies of each marker gene. When grown on PDA, the culture morphology of three of the transformants of (T×2, T×3 and T×4) was similar to the wild type. Four of the five transformants (T×3, T×4, T×21 and T×24) grew as well as the wild type on different media, and responded to changes in water potential in a similar manner to the wild type. All five transformants were equally parasitic on sclerotia of S. sclerotiorum compared with the wild type. Transformants T×3 and T×4 were the most similar to the wild type in biological characteristics and will be used in future studies. The results indicate that hph- and uidA-transformed strains of C. minitans will be useful for ecological studies on its survival and dissemination.