TheCalyptridium umbellatum complex (Portulacaceae), which has been treated as including from one to nine species, is shown to comprise three morphologically distinctive species. Two of these are common, wide-ranging, montane perennial herbs—C. umbellatum, which occurs over a large part of western North America, andC. monospermum, which largely replacesC. umbellatum in cismontane California. The third member of the complex is the rare or possibly extinctC. pulchellum. The perennial members of this complex are accorded the status of a separate genus,Spraguea, by most authors, but a comparative study of the morphology and karyology of all species ofCalyptridium shows this separation to be unwarranted. Chromosome numbers ofn = 22, obtained from eight populations ofC. umbellatum and six ofC. monospermum, constitute the first reported counts for these species. Hybridization studies involvingC. umbellatum andC. monospermum indicate that they are isolated by an incomplete noncrossability barrier.