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Atmospheric [CO.sub.2] and temperature may significantly modify plant production. Grasslands occupy in excess of 25% of the Earth's land area, but grassland species have received limited attention from researchers studying climate change. A 3-yr study was conducted to determine the effects of elevated atmospheric [CO.sub.2] and temperature on dry matter (DM) harvested from the [C.sub.3] legume `Florigraze' rhizoma peanut (RP, Arachis glabrata Benth.) and the Ca grass `Pensacola' bahiagrass (BG, Paspalum notatum Flugge). Both species were field grown in Millhopper fine sand (loamy siliceous Grossarenic Paleudult) in temperature-gradient greenhouses under different [CO.sub.2] (360 and 700 µmol [mol.sup.-1]) and temperature conditions (baseline [B], B+1.5, B+3.0, and B+4.5 ° C, where B equaled ambient temperature). Plots (2 by 5 m) were harvested three times in 1996 and four times each in 1997 and 1998. Analyzed across years, yield increased 25% for RP (P = 0.02) and tended to increase for BG (15%; P = 0.18) with the near doubling of [CO.sub.2], but there was species by [CO.sub.2] interaction (P = 0.06) as a result of the greater response to [CO.sub.2] by the [C.sub.3] legume. There was a positive effect of increasing temperature on yield of both species. Averaged across species, yield increased 11% in 1996, 12% in 1997, and 26% in 1998 as temperature increased from B to B+4.5 ° C. Under well-watered conditions in this experiment, elevated [CO.sub.2] increased DM harvested of a [C.sub.3] legume and tended to increase that of a [C.sub.4] grass, while the yield response to increasing temperature was positive for both species., ATMOSPHERIC [CO.sub.2] has steadily increased from preindustrial concentrations of 280 µmol [mol.sup.-1] and is predicted to double by the end of the 21st century (Barnola et al., 1994; Keeling and [...]