Plantago lanceolata and Trifolium repens were grown under ambient (400 μmol mol–1) and elevated (650 μmol mol–1) atmospheric CO2 conditions. Plants were inoculated with the arbuscular mycorrhizal fungus Glomus mosseae and given a phosphorus supply in the form of bonemeal. Six sequential harvests were taken in order to determine whether the effect of elevated CO2 on internal mycorrhizal colonization and external hyphal production was independent of the stimulatory effect of elevated CO2 on plant growth. At a given time, elevated CO2 increased the percentage of root length colonized (RLC), the total length of colonized root and the external mycorrhizal hyphal (EMH) density and decreased the ratio of EMH to total length of colonized root. When plant size was taken into account, the CO2 effect on RLC and total length of colonized root was greatly reduced (and only apparent for early harvests in T. repens) and the effects on the EMH parameters disappeared. Root tissue P concentration was unchanged at elevated CO2, but there was a decrease in shoot P at the later harvests. There was no direct effect of elevated CO2 on P inflow for the earlier period (< 50 d) of the experiment. However, over the last period, there was a significant negative effect of elevated CO2 on P inflow for both species, independent of plant size. It is concluded that elevated CO2 had no direct effect on mycorrhizal colonization or external hyphal production, and that any observed effects on a time basis were due to faster growing plants at elevated CO2. However, for older plants, elevated CO2 had a direct negative effect on P inflow. This decrease in P inflow coincides with the observed decrease in shoot P concentration. This is discussed in terms of downregulation of photosynthesis often seen in elevated CO2 grown plants, and the potential for mycorrhizas (via external hyphal turnover) to alleviate the phenomenon. The direction for future research is highlighted, especially in relation to carbon flow to and storage in the soil.