SUMMARYThe growth of wheat seedlings which were already mycorrhizal when transplanted to a field deficient in phosphorus was improved compared with non‐mycorrhizal controls, and grain yield was increased three‐fold by the fungus, indicating thatEndogonestimulated growth and increased yield. Differences between mycorrhizal and non‐mycorrhizal wheat were eliminated by the application of phosphate fertilizer, indicating that the fungus does not enhance wheat growth in soils containing enough available phosphate. It is probable that the mycorrhizal effect is primarily to improve the supply of phosphate. There were clear relationships between spore number in the soil and mycorrhizal development and between the extent of root infection and increased growth. The extent of root infection was greatest in mycorrhizal plants in soil not supplemented with phosphate and it decreased in inoculated plants in the plot supplemented with superphosphate.The non‐centrospermous and non‐zygophyllaceous weeds growing on the experimental field had typical vesicular arbuscular infection and indigenousEndogonespores in their rhizospheres. The centrospermous plants were non‐mycorrhizal and had noEndogonespores in their r