Experiments were conducted in the field and the greenhouse to determine whether vesicular-arbus-cular mycorrhizae affect growth and competition between the native perennial Stipa pulchra and the introduced annual Avena barbata. Soils in the greenhouse were steam-sterilized, and in the field they were treated with the fungicide benomyl. Stipa pulchra showed decreased shoot dry mass and increased root mass when inoculated, while A. barbata showed the opposite response, increased shoot mass and decreased root mass. Mycorrhizal A. barbata also produced more seeds. Mycorrhizae did not alleviate the negative effects of competition of A. barbata on S. 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 native S. pulchra, the original fungal species composition may return relatively quickly. Where A. barbata dominates, inoculation with mycorrhizal fungi alone will not suffice for establishing S. pulchra, and the usual practices for control of weed competition need to be employed.