The presence of intraradical vesicles increases the inoculum potential of excised mycorrhizas. High infectivity was obtained with root pieces colonized by species of vesicular-arbuscular (VA) mycorrhizal fungi that formed intraradical vesicles (i.e. Glomus fasciculatum, Glomus mosseae and Acaulospora spinosa). Root pieces colonized by mycorrhizal fungi which generally do not form intraradical vesicles (i.e. Gigaspora margarita and Gigaspora gigantea) were not infective. Extraradical vesicles of Gigaspora margarita were also not infective propagules. Intraradical vesicles separated from the colonized roots remained highly infective but hyphae and cell debris from the same roots were not. Mycorrhizal fungi in root pieces treated for short times in sodium hypochlorite were apparently killed.