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SUMMARYA large, yellow, one-celled spore 291–812μ in diameter was found to be common in Illinois soils. Inoculation studies showed that it causes a mycorrhizal infection of the phycomycete type. The appearance of the spore and attached hyphae suggest that it is a sexual stage. These spores have been separated in quantity from soil by wet sieving and decanting. They have been surface sterilized and pure cultures grown from them. Spores germinated by producing germ tubes. Hyphae varied from coarse, irregular and coenocytic to fine, smooth and regularly septate. It was grown on various types of agar media, but only a limited growth, not exceeding 22 mm in length, was obtained. Somewhat better growth was obtained in water culture with non-sterile soil organic matter or bits of hemp seed.When artificial inoculations were made, hyphae from these spores penetrated plant roots. Hyphae were intracellular as they passed through the outer cortex and when they reached the inner cortex they became intercellular as they grew through the root. Arbuscles were produced in cortical cells just outside the endodermis. Hyphae within the roots were non-septate. After penetration, tight bunches of echinulate spores formed on the external mycelium. The internal and external vesicles often associated with this type of mycorrhiza in the field have not formed in artificial inoculations.Cross inoculation experiments utilizing spores from known sources indicate a wide host range. What appeared to be the same strain infected roots of red clover, corn, strawberry, and sweet clover.