Prunella vulgaris is was inoculated with different arbuscular mycorrhizal fungi (AMF) and grown at two concentrations of CO2 (ambient 350 mu l l(-1) and elevated 600 mu l l(-1)) to test whether a plants response to elevated CO2 is dependent on the species of AMF colonizing the roots. Using compartments accessible only to AMF hyphae but not to roots we also tested whether elevated CO2 affects the growth of external AMF hyphae. Plant biomass was significantly greater at elevated than at ambient CO2; the biomass of the root system for example increased by a factor of 2. The colonization of AMF inside the root remained constant indicating that the total AMF inside the root system also increased by a factor of 2. The length of external AMF hyphae at elevated CO2 was up to 5 times that at ambient CO2 indicating that elevated CO2 promoted allocation of AMF biomass to the external hyphae. The concentration and content of phosphorus in the stolons differed significantly between ambient and elevated CO2 but this resulted in either an increase or a decrease according to which AMF isolate occupied the roots. We hypothesized that an increase in external hyphal growth at elevated CO2 would result in increased P acquistion by the plant. To test this we supplied phosphorus in a compartment only accessible to AMF hyphae. Plants did not acquire more phosphorus at elevated CO2 when phosphorus was added to this compartment. Large increases in AMF hyphal growth could however? play a significant role in the movement of fixed carbon to the soil and increase soil aggregation.