Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol−1) and elevated (700 μmol mol−1) CO2 in either the presence or absence of the root hemi-parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO2 alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO2 was 35% that of uninfected, control plants, while at elevated CO2, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO2 supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO2. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO2. There were significant CO2 and Striga effects on photosynthetic metabolism and instantaneous water-use efficiency of O. sativa. The response of photosynthesis to internal [CO2] (A/Ci curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO2 had significantly lower CO2 saturated rates of photosynthesis, carboxylation efficiencies and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 188.8.131.52) contents than uninfected, ambient CO2-grown O. sativa. In contrast, infection with S. hermonthica prevented down-regulation of photosynthesis in O. sativa grown at elevated CO2, but had no impact on photosynthesis of hosts grown at ambient CO2. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO2 had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO2. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO2 or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO2 concentrations. These results demonstrate that elevated CO2 concentrations can alleviate the impact of infection with Striga on the growth of C3 hosts such as rice and also that infection can delay the onset of photosynthetic down-regulation in rice grown at elevated CO2.