Five species of endomycorrhizal fungi differed in their ability to stimulate phosphorus uptake and growth of subterranean clover when inoculated into two untreated field soils which were phosphate deficient. Each soil contained a different indigenous endomycorrhizal fungus. The amount of roots converted to mycorrhizas by each indigenous and inoculant fungus was estimated by using differences in the morphology of the infection within the roots and a line-intercept method. Effectiveness of the inoculant fungi in increasing plant growth was related to the infectivity of the fungi from the inocula used. The fungi indigenous to the two soils used did not appear to affect the extent to which mycorrhizas were formed by any of the inoculant fungi.