Material for this study consisted of collections of water mites made by Dr. R. M. Muttkowski in Lake McDonald, Glacier National Park, Montana, during the months of July and August, 1925. The author wishes here to express her thanks to Dr. Muttkowski for the opportunity to examine this material and thus to add a little to our scanty knowledge of the Hydracarina of the Rocky Mountain region. Nearly five hundred individuals were secured; these represented seven genera and ten species, one of which is new and one undetermined. About three fourths of the number were Pionas and Limnesias, each genus with two species. The number of species is therefore low, and most of them are common farther east. The size of the individuals was notable; many specimens reached the maximum length recorded for the species.