The rate of photorespiration in several grass species was shown to be highly variable and dependent on the species, genotype, and conditions under which the plants were grown. Photorespiration, measured as oxygen uptake, was negligible in Cenchrus ciliaris and Paspalum dilatatum but significant in Lolium spp. and Festuca arundinacea. There were significant differences in the rate of photorespiration among plants within a Lolium population of diverse origin and these differences were independent of the conditions under which the plants were grown. Among the temperate grasses there was a significant correlation between photorespiration and the CO2-compensation concentration and both parameters were very low in P. dilatatum. Plants grown in day/night temperatures of 15/10 °C compared with 25/20 °C had faster rates of dark respiration but slower rates of light respiration when measured at the same temperature. Photorespiration was faster than dark respiration although differences in respiration among plants in the light were not shown in the dark.