Distributional patterns of Microtus montanus and Microtus pennsylvanicus in relation to various plant communities and to characteristics of habitats were studied in the vicinity of Bozeman, southwestern Montana, in 1968 and 1969. A description of the physiography and vegetation of nine community types is given. During a total of 17,700 trap-nights, 762 M. montanus and 583 M. pennsylvanicus were snap-trapped from 59 study plots. These two species occurred sympatrically in 31 of the 59 sample areas. Preferred habitat of M. pennsylvanicus is in moist areas where grasses, especially Poa pratensis, and grass-like species are dominant plants, comprising 50 per cent or more of the vegetation by canopy coverage, and total canopy cover of all herbaceous material is at least 85 per cent. The preferred habitat of M. montanus is not as well delineated as that of M. pennsylvanicus. Microtus montanus appeared to have a wider ecological tolerance than M. pennsylvanicus, and demonstrated a direct general correlation between abundance and the dryness of the substrate.
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