Mimulus glabratus Kunth is a widespread, aquatic perennial of the Scrophulariaceae most common in the western United States. It is characterized by an attractive, yellow, bilabiate corolla; a cup-shaped calyx; two sensitive, plate-like stigmas; didynamous stamens; and opposite, ovate to broadly rounded leaves with entire to dentate margins. The lower lip of the corolla is variously brown-spotted and the lower leaves tend to have a well developed petiole. The species ranges from western Quebec Province to Saskatchewan and south through Mexico to southern Chile and grows in shallow water along streams and lake shores. The plants are prostrate to erect herbs which root freely at their lower nodes and tend to form geographically isolated groups of fifty to several hundred shoots. Seven varieties has been recognized; some are widespread and others seem to be narrow endemics. The main objective of this work is to determine whether or not the two varieties in Michigan are indeed distinct. One variety is the widespread M. glabratus var. fremontii (Benth.) A. L. grant which ranges from western Quebec Province to Saskatchewan Province and south to Mexico. The other is M. glabratus var. michiganensis (Pennell) Fassett, a Michigan endemic found only in the Mackinac Stratis region (Cheboygan, Mackinac, Charlevoix, and Emmet Counties) and in Leelanau and Benzie Counties. Pennell (1935) accorded the latter subspecific recognition due to its larger flower, more sinuate-dentate leaves, unspotted corolla, and a more upright habit than M. glabratus var. fremontii. Alternatively, Fassett (1939) considered it a variety. However, an examination of herbarium specimens by myself and others interested in the flora of Michigan suggested that a continuum of morphological diversity between the two varieties may exist.