Pyrenophora semeniperda is a generalist fungal pathogen that occurs primarily on monocot seed hosts. It is in the phylum Ascomycota, which includes both self-compatible (homothallic) and self-incompatible (heterothallic) species. Homothallic fungal species contain complementary mating-type (MAT) idiomorphs in a single unikaryotic strain, while heterothallic strains contain a single MAT idiomorph requiring interaction between strains of complementary mating-types for sexual reproduction to occur. Because the majority of P. semeniperda strains contained either MAT1 or MAT2, this species was provisionally categorized as heterothallic. However, many strains contain both MAT idiomorphs and appear to be homothallic. These results warranted a closer look at the MAT idiomorphs and the structure of the P. semeniperda genome in order to assure accurate characterization of the MAT locus. Additionally, an assessment of the geographic distribution of MAT idiomorphs provides us with insight into the genetic diversity of P. semeniperda and the reproductive strategies that it employs. In this study, we characterized the P. semeniperda MAT locus and assessed the idiomorph distribution of 514 isolates from 25 P. semeniperda populations collected from infected Bromus tectorum (cheatgrass) seeds. Additionally, we used simple sequence repeat (SSR) and MAT idiomorph length polymorphisms to demonstrate the existence of dikaryotic strains and pseudohomothallism in this fungus. We identified a unique variable number tandem repeat (VNTR) within each idiomorph of the MAT locus of P. semeniperda. Presence of the VNTR in all MAT loci analyzed from strains collected in the Intermountain West suggests ancient proliferation of this repeat. The persistence and effectiveness of P. semeniperda strains in the cheatgrass pathosystem depend not only on the density of the fungus in the soil, but also on the genetic heterogeneity of each population. Our study suggests that P. semeniperda genetic diversity is increased both through MAT locus-dependent sexual reproduction and asexually through anastomosis.