Seedlings (70-d-old) of two tall fescue (Festuca arundinacea Schreb.) genotypes, heat-tolerant Jaguar 3 and heatsensitive TF 66, were exposed to a high temperature stress of 35/30 °C (day/night) for 20 d and both light-saturated and CO 2 -saturated leaf stomatal conductance decreased, especially in TF 66. Higher reductions of quantum efficiency, carboxylation efficiency and maximum photochemical efficiency of photosystem 2 in dark adapted leaves (measured as F v /F m ) occurred in TF 66 than in Jaguar 3. High temperature stress increased photorespiration in the two plants, but more in TF 66. Moreover, high temperature stress also reduced the growth, chlorophyll content and caused cell membrane injuries in the two cultivars, the changes were again more pronounced in TF 66 than in Jaguar 3.