Eragrostis pilosa (Linn.) P Beauv., a C4 grass native to east Africa, was grown at both ambient (350 μmol mol−1 and elevated (700 μmol mol−1) CO2 in either the presence or absence of the obligate, root hemi-parasite Striga hermonthica (Del.) Benth. Biomass of infected grasses was only 50% that of uninfected grasses at both CO2 concentrations, with stems and reproductive tissues of infected plants being most severely affected. By contrast, CO2 concentration had no effect on growth of E. pilosa, although rates of photosynthesis were enhanced by 30–40% at elevated CO2. Infection with S. hermonthica did not affect either rates of photosynthesis or leaf areas of E. pilosa, but did bring about an increase in root[ratio ]shoot ratio, leaf nitrogen and phosphorus concentration and a decline in leaf starch concentration at both ambient and elevated CO2. Striga hermonthica had higher rates of photosynthesis and shoot concentrations of soluble sugars at elevated CO2, but there was no difference in biomass relative to ambient grown plants. Both infection and growth at elevated CO2 resulted in an increase in the Δ13C value of leaf tissue of E. pilosa, with the CO2 effect being greater. The proportion of host-derived carbon in parasite tissue, as determined from δ13C values, was 27% and 39% in ambient and elevated CO2 grown plants, respectively. In conclusion, infection with S. hermonthica limited growth of E. pilosa, and this limitation was not removed or alleviated by growing the association at elevated CO2.