The phytopathogenic fungus Drechslera tritici-repentis causes tan spot, an important disease of wheat in the southern Brazilian state of Rio Grande do Sul. Twelve D. tritici-repentis isolates were obtained from wheat seeds from different locations in the state. Their colony morphology on potato dextrose agar and polymorphisms in genomic DNA by the random amplified polymorphic DNA (RAPD) method were investigated. For the RAPD method, 23 primers were tested of which nine were selected for use in the study of D. tritici-repentis polymorphisms. The degree of similarity between isolates was calculated using a simple matching coefficient and dendrograms constructed by the unweighted pair-group method with arithmetical averages (UPGMA). The morphological and RAPD analyses showed intraspecific polymorphisms within the isolates, but it was not possible to establish a relationship between these polymorphisms and the geographical regions from where the host seeds were collected.