Five cool-season C3 pasture species (Bromus willdenowii Kunth, Festuca arundinacea Schreb, Lolium multiflorum Lam., Lolium perenne L. and Trifolium repens L.) were grown at a constant temperature of 12°C to assess effects of elevated CO2 on growth and photosynthesis. Plants were grown at either 350 or 700 'mol mol'1 CO2 for 42-56 d, after which they were destructively harvested to assess biomass. At the same time, leaf gas exchange was measured at 12°C for each species at both CO2 concentrations. Photosynthetic response to CO2, that is the A/Ci curve, was also measured. Results showed all species responded to elevated CO2 at low temperature by increasing total biomass, relative to that at ambient CO2, from 105 to 158%. T. repens and B. willdenowii had the smallest response, and the two Lolium species had the greatest response. Photosynthesis also responded strongly to elevated CO2 at low temperature, increasing on average by 40%. Only small growth CO2 effects occurred with the A/Ci curves, with lower CO2-saturated photosynthesis only for L. perenne and T. repens grown at elevated CO2. The photosynthetic response to CO2 was not correlated with the growth response. However, the product of photosynthesis and leaf area, that is carbon supply, was highly correlated with the growth rate, and growth CO2 had a marked effect. Results suggest T. repens responded poorly to elevated CO2 at low temperature because of a low capacity to fix carbon, while both Lolium species responded strongly at low temperature because of a high capacity to fix CO2. Changes to photosynthetic model parameters are now required to account for growth temperature effects on the CO2 response.