Premise of the study: Microsatellite markers were isolated and characterized in Mimulus ringens (Phrymaceae), a herbaceous wetland perennial, to facilitate studies of mating patterns and population genetic structure. Methods and Results: A total of 42 polymorphic loci were identified from a sample of 24 individuals from a single popula- tion in Ohio, USA. The number of alleles per locus ranged from two to nine, and median observed heterozygosity was 0.435. Conclusions: This large number of polymorphic loci will enable researchers to quantify male fitness, patterns of multiple pa- ternity, selfing, and biparental inbreeding in large natural populations of this species. These markers will also permit detailed study of fine-scale patterns of genetic structure.