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Habitat alteration by exotic plant species can have profound effects on vertebrates, but its effects on invertebrates are less well-known. Crested wheatgrass (Agropyron cristatum (L.) Gaertn.) is a perennial grass that has been planted on >10(6) ha of the Great Plains. We tested the hypothesis that invertebrate communities (especially ants) differed between native grasslands and A. cristatum stands, using pitfall traps in Saskatchewan and Montana. Ant species composition differed significantly between native grasslands and A. cristatum stands, but there were no differences in total ant abundance, the abundance of functional groups, or species richness. Ant species richness was significantly greater in Montana than Saskatchewan. In Saskatchewan, bare ground was positively related to total ant abundance and the abundance of "cold-climate specialist" and "opportunist" functional groups of ants. In Montana, the cover of forbs was positively related to total ant abundance. The abundances of individual ant species were not predicted by any vegetation characteristics, except for Formica obscuripes Forel, 1886, which increased significantly with litter. The total abundance of other invertebrates was greater in native grasslands than in A. cristatum stands, although not significantly so. Within each vegetation type, variation in ant communities may depend either directly on the effects of vegetation species composition, or indirectly via the effect of vegetation on other factors such as temperature. The results suggest that ant community composition was influenced more by variation within grasslands and between locations than by differences between native and exotic grasslands.