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Bromus tectorum (cheatgrass), an alien annual grass, has become naturalized in a variety of habitats ranging from arid steppe to mesic forest communities within the intermountain region of western North America. To examine the contribution of genetic differentiation to the success of cheatgrass, we measured photosynthesis, growth, phenology and biomass allocation in populations collected as seed from contrasting habitats and grown in a glasshouse. In addition, we examined the effects of drought and population seed source on characteristics of photosynthesis and growth. In general, leaf-level photosynthetic response to variation in light, temperature and soil moisture did not vary significantly among the different habitat populations. Water-use efficiencies were not detectably different among populations. Growth, phenology and biomass allocation were also relatively insensitive to variation in soil moisture induced by the watering treatments. In contrast, phenology and biomass allocation exhibited marked variation among populations. Populations from arid steppe sites flowered earlier, set seed earlier and allocated less biomass to root growth than did populations from mesic steppe or forest habitats. Higher biomass allocation to shoot growth in arid steppe populations allows a plant to reach reproductive size more rapidly and set seed before moisture becomes limiting in late spring. These results suggest that mechanisms of adaptation such as drought avoidance may be better understood by focusing on whole-plant phenology and allocation patterns rather than on the physiology of a particular organ.