AbstractExperiments were conducted in the field and the greenhouse to determine whether vesicular‐arbus‐cular mycorrhizae affect growth and competition between the native perennialStipa pulchraand the introduced annualAvena barbata.Soils in the greenhouse were steam‐sterilized, and in the field they were treated with the fungicide benomyl.Stipa pulchrashowed decreased shoot dry mass and increased root mass when inoculated, whileA. barbatashowed the opposite response, increased shoot mass and decreased root mass. MycorrhizalA. barbataalso produced more seeds. Mycorrhizae did not alleviate the negative effects of competition ofA. barbataonS. pulchra, as has been demonstrated for other pairs of weedy and nonweedy species. The same three species of mycorrhizal fungi were present in annual and perennial grasslands, but their relative composition was different. When inoculum from the two grassland types were tested in the field, the fungal species began to revert within five months to the species composition found in grasslands of the host plant. This indicates that, once annual grassland has been revegetated with the nativeS. pulchra, the original fungal species composition may return relatively quickly. WhereA. barbatadominates, inoculation with mycorrhizal fungi alone will not suffice for establishingS. pulchra, and the usual practices for control of weed competition need to be emp