The rotifer Keratella armadura n.sp. is described from a shallow alkaline bog lake from Michigan’s lower peninsula. The facet pattern of the dorsal plate is similar to that of the common North American phenotype of Keratella cochlearis and K. taurocephala. The rigid, thickened lorica, long recurving posterior spine, and constriction of the body near the base of the anterior spines is distinctive from known congenors. The species was abundant (>330 individuals/L) within a well-defined habitat, and appears to have a narrow geographic range. Keratella armadura occurred in spring and summer months and was absent from fall and winter collections. This restricted distribution, if common among rotifers, suggests that many undescribed species still exist in Michigan and in North America.