Roots of Opuntia, polyacantha Haw. from Juab County, Utah were found to be nodulated. An anatomical study of the nodulated roots showed them to be ectotrophic mycorrhizae. Fusarium poae (Peck) Wollenweaver was found associated with the nodules both as an epiphyte and an endophyte. However, reinfection with this organism failed to induce subsequent nodulation. Under certain conditions the roots of o. polyacantha are able to overcome the mycorrhizal infection and elongate. It is not uncommon for several branch roots to originate from a single nodule. It is suggested that the fungus-root association found in o. polyacantha represents an early stage in the evolution of ectotrophic mycorrhizae.