Growth, competitive performance, and physiological response of the introduced annuals downy brome (Bromus tectorum L.) and medusahead (Taeniatherum asperum (Sim.) Nevski) were compared under various environmental conditions in growth chambers and greenhouses. Seeds of both species were collected near Verdi, Nevada. Effects of soil moisture on growth, resource partitioning, photosynthesis, stomatal conductance, and transpiration rates were characterized for both species. Competitive interactions between the species were assessed in replacement series experiments. Under unlimited moisture, mean dry matter production and leaf area were greater in downy brome than medusahead. Height and root/shoot ratios were higher in medusahead or similar to those of downy brome. Mean values of relative growth rate (RGR) and net assimilation rate (NAR) over the entire growing period were similar between the species. RGR was significantly higher in downy brome of <40 d age, and higher in medusahead of >40 d age. Rate of biomass allocation to leaf area (LAR) correlated best with dry matter production in both species. LAR was higher in downy brome than medusahead on most sample dates. Moisture stress reduced height, leaf development, and consequently biomass production in both species. Downy brome maintained higher dry matter production than medusahead when moisture was limited, due to higher LAR in downy brome. Stomatal conductance and transpiration rates were higher in medusahead than downy brome throughout the growing period. Under moisture stress, both species were similar in conductance and transpiration rates. Carbon fixation response (measured by IRGA in laboratory cuvettes) to light throughout the growing period were similar for the two species, regardless of moisture conditions. Moisture stress reduced photosynthesis in each species by > 50%. Competitive performance in mixtures was related to the maximum relative growth rate (Rmax) attained during early plant development. With unlimited moisture, downy brome had higher Rmax and LAR than medusahead. These differences gave competitive superiority to downy brome in interspecific mixture. Under moisture stress, both species were similar in Rmax and in general competitive performance. Effects of three temperature regimes and two moisture levels on growth, interference, and photosynthesis response in downy brome and medusahead were investigated in growth chamber study. Plants were grown separately and in intra- and interspecific pairs. Optimum temperature for growth of both species was the moderate temperature regime (24/11 C). At this moderate regime, medusahead yield was higher than that of downy brome. Downy brome yield was less affected by temperature extremes and was significantly higher than that of medusahead at these extremes. Moisture limitation reduced yield in both species at all temperature regimes. Variation in performance between species at each temperature regime was closely related to photosynthetic response and pattern of resource allocation. In mixtures, downy brome had higher yield than medusahead at all temperature regimes. At low temperature (16/5C), both species were competitively similar. The competitive advantage of downy brome over medusahead increased with increasing temperature and was most profound at high temperature (32/16C). In limited moisture, downy brome maintained higher yield than medusahead in pure stands and mixtures. Species competitive performance in mixture also correlated with photosynthesis and resource allocation. Effects of nitrogen and phosphorus supply on growth and interference between downy brome and medusahead were studied in sand culture using a modified replacement series experiment in the greenhouse. Three levels of nitrogen and phosphorus were applied in all possible combinations. Height, number of leaves, and biomass production were reduced in both species in response to nutrient reduction. Downy brome had higher dry matter production than medusahead at high concentrations of nitrogen or phosphor us or when both nutrient elements were present. When either nutrient was lacking, yields of both species were similar. Nitrogen deficiency was more influential on yield reduction than phosphorus in both species. In mixtures, downy brome was highly competitive at high concentrations of N and P. At lower levels of N and P, the species performed as competitive equals. Thus, early patterns of resource allocation and high growth rates in downy brome enable that species to outperform medusahead at high nutrient concentrations. Findings of these studies were discussed in relation to the distribution of the two species in the field.