A previous report indicated that insertion of the carboxin resistance (cbx(R)) gene into the Ustilago maydis genome impaired the pathogenic ability of the fungus towards Zea mays, the corn host. Because we had anecdotal evidence from work in our laboratory that this was not necessarily the case we decided to determine how general was the observation of reduced pathogenicity associated with cbx(R). To accomplish this we tested the pathogenicity of several strains that had been transformed with the cbx(R) gene and compared them with non-transformed strains or strains transformed with the gene conferring hygromycin resistance which is a commonly used selectable marker in this fungus. Our results indicate that carboxin resistance does not significantly alter pathogenicity and is therefore a suitable marker for use in genetic analysis of U. maydis.