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Previous studies have shown that both photoinhibition and low temperature impairment of leaf development occur in C 4 species growing in temperate climates. These result in reductions in the maximum quantum efficiency ( ?) and the light-saturated rate of CO 2 uptake (A sat ). The perennial C 4 -grass Miscanthus x giganteus has been shown to attain high productivity in northern Europe. This study examines and analyses the seasonal variation in photosynthetic gas exchange of M. x giganteus grown in southern England. Although the crop was exposed to chilling temperatures at the start of the season neither A sat nor ? were impaired, when assessed at a measurement temperature of 24°C. Between May and July the mean values of A sat and ? were 34 μmol m -2 s -1 and 0.067, respectively, declining to 20 μmol m -2 s -1 and 0.045 in October. In the field, on clear days between May and July, the typical midday rates of CO 2 uptake ranged between 20-27 μmol m -2 s -1 , although in late-June a peak mean value of 35 μmol m -2 s -1 was attained. The study shows that under the cool temperate conditions of southern England, M. x giganteus, unlike all C 4 species previously examined, is able to realize the photosynthetic potential of the C 4 process without suffering any apparent low temperature impairment, except possibly at the very end of the growing season.