Multilocus interactions (also known as Dobzhansky-Muller incompatibilities) are thought to be the major source of hybrid inviability and sterility. Because cytoplasmic and nuclear genomes have conflicting evolutionary interests and are often highly coevolved, cytonuclear incompatibilities may be among the first to develop in incipient species. Here, we report the discovery of cytoplasm-dependent anther sterility in hybrids between closely related Mimulus species, outcrossing M. guttatus and selfing M. nasutus. A novel pollenless anther phenotype was observed in F₂ hybrids with the M. guttatus cytoplasm (F_2G) but not in the reciprocal F_2N hybrids, F₁ hybrids or parental genotypes. The pattern of phenotypic segregation in the F_2G hybrids and two backcross populations fit a Mendelian single-locus recessive model, allowing us to map the underlying nuclear locus to a small region on LG7 of the Mimulus linkage map. Anther sterility was associated with a 20% reduction in flower size in backcross hybrids and we mapped a major cytoplasm-dependent corolla width QTL with its peak at the anther sterility locus. We argue that the cytonuclear anther sterility seen in hybrids reflects the presence of a cryptic cytoplasmic male sterility (CMS) and restorer system within the hermaphroditic M. guttatus population and therefore name the anther sterility locus restorer-of-male-fertility (RMF). The genetic mapping of RMF is a first step toward testing hypotheses about the molecular basis, individual fitness consequences, and ecological context of CMS and restoration in a system without stable CMS-restorer polymorphism (i.e., gynodioecy). The discovery of cryptic CMS in a hermaphroditic wildflower further suggests that selfish cytoplasmic evolution may play an important, but often undetected, role in shaping patterns of hybrid incompatibility and interspecific introgression in plants.