The gene encoding ornithine decarboxylase (ODC) from Ustilago maydis was cloned. A conserved PCR product amplified from U. maydis DNA was synthesized and used to screen a genomic library of the fungus. Alignment of its deduced protein sequence with those of other cloned ODCs showed a high degree of homology. Gene replacement was obtained by removal of a central part of the gene and insertion of the hygromycin resistance cassette. The null mutant thus obtained displayed no ODC activity and behaved as a polyamine auxotroph. This result is evidence that a single ODC gene exists in the fungus, and that U. maydis utilizes the ODC pathway as the only mechanism for polyamine biosynthesis. When grown in polyamine-containing media, the null mutant accumulated a polyamine pool which further sustained its normal rate of growth in polyamine-free media for approximately 12-16 h. When putrescine concentrations lower than 0.5 mM were employed, the mutant grew at a normal rate but was unable to engage in the dimorphic transition. Under conditions favourable for mycelial growth, the mutant grew with a yeast-like morphology in liquid media, and formed smooth colonies consisting of yeast cells on solid media. Reversion to normal dimorphic phenotype required high concentrations of putrescine or spermidine. These results are evidence that concentrations of polyamines higher than those necessary to sustain vegetative growth are required for the dimorphic transition in U. maydis.