Abstract.Net photosynthesis and transpiration rates of potato plants, grown in pots in the greenhouse, were measured at various light irradiances and ambient CO2concentrations, 3d after inoculation with second stage juveniles ofGlobodera pallida.Gas exchange rates, both in darkness and in light, and the initial light use efficiency were strongly reduced by nematodes. Stomatal conductance of infected plants was lower than that of control plants and showed little response to decreasing ambient CO2concentration. The maximum internal CO2concentration of infected plants was lower than that of control plants.Globodera pallidareduced photosynthesis also by apparent non‐stomatal effects.The effects ofG. pallidaon gas exchange rates are similar to the effects of abscisic acid in the transpiration stream and of abiotic stresses in the root environment. Apparently, there is a general response of plant roots to adverse conditions. The reduction of photosynthesis may be an important factor in yield reduction by potato cyst nematode