A new paleovegetation reconstruction based on both phytolith and paleosol carbon isotopic data is presented for the middle–late Miocene (11.2–9.5 Ma) of southwestern Montana. Both sources of data indicate an open-habitat, C₃ grassland ecosystem with a small C₄ component (< 10%). However, while a penecontemporaneous record from further south within the same region indicated significant spatial and temporal variability in C₄ abundance, the new record indicates relatively little change either in space or in time. Because paleoclimatic conditions reconstructed at these two sites are the same, we hypothesize that other ecological factors controlled C₄ abundance across the region. The relative abundance of C₃ and C₄ plants in the Miocene is the same as in modern environments at both sites, suggesting that their distribution was likely established by the middle–late Miocene. This is consistent with the idea that paleoelevation was also similar, which agrees with independent constraints on the regional tectonic history. The Montana records indicate a more limited distribution of C₄ vegetation than is observed in the Great Plains and elsewhere globally, as well as variable timing in the shift to C₄ dominance in ecosystems. Taken together, this local, regional, and global variability during a time of little or no climate change suggests that C₄ distribution and expansion was driven by local, rather than global, environmental factors.