Sorghum plants were grown in the laboratory with the root system of each plant split between two pots. Three split pot treatments were established: (− −) treatment, where both halves of the root were free from Striga; (− +) treatment, where the soil in one half of the pot had been inoculated with Striga seed; (+ +) treatment, where the soil in both halves of the pot had been inoculated with Striga. Seed, stem and leaf weight were reduced by 82, 60 and 26 per cent respectively in (+ +) plants compared to (− −) plants. Partially infected plants (− +) behaved similarly to (+ +) plants. Rates of light saturating carbon dioxide fixation in (+ +) and (− +) plants were only 60 per cent of those measured in (− −) plants. This reduction was independent of changes in stomatal conductance. The effects of Striga on the growth and photosynthesis of sorghum appear to be independent of the degree of parasitism to which the host is subjected. The difference in production between infected and uninfected plants was greater than could be accounted for in term of competition with the parasite for resources, and Striga appears to have a pathological effect on the host.