Mimulus glabratus var. michiganensis (Pennell) Fassett, an endemic of northern Michigan, is morphologically distinct from its sympatric relative, M. glabratus var. fremontii (Benth.) A. L. Grant. Pistil and style lengths are the best morphological characters to distinguish the two varieties. Mimulus glabratus var. michiganensis is primarily a vegetative apomict; its percentages of viable pollen and seed set are significantly lower than those of M. glabratus var. fremontii. Previous chromosome counts for both varieties (n = 15) are confirmed; however, microspores of M. glabratus var. michiganensis with 14 chromosomes are found. Mimulus glabratus var. michiganensis may have originated from chromosomal rearrangements of individuals of M. glabratus var. fremontii, as the hybrid of the variable M. guttatus D. C. (n = 14) and M. glabratus var. fremontii, or as aneuploid populations of M. guttatus.