Goatsrue, a perennial legume, is an exotic noxious weed currently found in the United States only in Cache County, Utah. It infests irrigated pastures, alfalfa fields, and noncropland areas. In order to compare their responses to temperature, goatsrue and alfalfa were grown in artificially illuminated controlled-environment chambers in 16 day/night temperature regimes ranging from 15/4 to 36/25 C. Growth analysis was used to evaluate effects of temperature on dry matter accumulation, leaf area production, and biomass allocation. Both species grew best at day/night temperatures of 22/25, 29/ 18, and 29/25 C. Leaf appearance rates were linearly related to mean daily temperature. Goatsrue produced fewer but larger leaves and a greater total leaf area than alfalfa. Biomass partitioning to leaves was greater in goatsrue, whereas partitioning to stems was greater in alfalfa. Response of vegetative dry matter production to temperature closely paralleled response of leaf area duration in both species. Alfalfa generally had a higher net assimilation rate, but the greater leaf area duration of goatsrue resulted in greater dry matter accumulation in this species after 50 d of growth. Overall responses to temperature were similar in the two species. Thus it seems likely that goatsrue could become a much more widely distributed weed in alfalfa.