We have demonstrated that genes from Ustilago maydis can be cloned by direct complementation of mutants through the use of genomic libraries made in a high-frequency transformation vector. We isolated a gene involved in amino acid biosynthesis as an illustrative example and showed that integrative and one-step disruption methods can be used to create null mutations in the chromosomal copy of the gene by homologous recombination. The results of this investigation make it clear that one-step gene disruption will be of general utility in investigations of U. maydis, since simple, precise replacement of the sequence under study was readily achieved.