Sites on different strata of Bearpaw shale and on alluvium derived from the shale in a small basin in northeastern Montana supported strikingly different plant communities, including three Nuttall saltbush, three big sagebrush, two greasewood, one western wheatgrass, one blue grama, one silver sagebrush, one foxtail, one buckwheat, and a community of mixed shrubs. Several soil factos were measured, but only total soil—moisture stress and soil—moisture volume gave rational ordinations of the communities studied. Total soil—moisture stress at the average root depth ranged from a high of 96 bars for a Nuttall saltbush community to only 19 bars for a mixed shrub community. The Nuttall saltbush community was found on soils having a high soluble salt content, high soluble sodium percentages, high total soil—moisture stress, and low infiltration rates. Soils at the big sagebrush site had low soluble salt content, relatively high soluble sodium, and intermediate total soil—moisture stress values. The wettest site, subject to spring flooding, was occupied by western wheatgrass. Quantities of water evapotranspired from each habitat (calculated as maximum soil moisture minus minimum plus increments added to soils by summer storms) when related to precipitation provided approximations of runoff from, or run—in moisture for, each habitat.