Sporocarps of 2 vesicular-arbuscular mycorrhizal fungi, Glomus epigaeus and G. fasciculatus, often contain hyperparasitized spores even when produced in greenhouse pot cultures. Two of these hyperparasites were isolated on agar media and identified as Anguillospora pseudolongissima and Humicola fuscoatra. Glomus spores parasitized by A. pseudolongissima contained swollen sausage-shaped hyphae which are constricted at the septa, while those parasitized by H. fuscoatra contained slender hyphae and/or aleurospores. G. epigaeus spores, visibly free from parasites, when added to water agar or autoclaved soil containing A. pseudolongissima became 51.4 and 69.3% parasitized, respectively, while H. fuscoatra parasitized 48.2 and 70.9%, respectively. Similarly, G. fasciculatus became 67.1 and 94.6% parasitized by A. pseudolongissima and 56.8 and 91.2% parasitized by H. fuscoatra, respectively. Control spores of G. fasciculatus or G. epigaeus placed in either soil or on agar without parasitic fungi were only 1.3-3.0% parasitized. Of 9 fungicides tested, only mancozeb reduced growth of both hyperparasites without inhibiting the germination of mycorrhizal fungi. A 3rd chytridlike hyperparasite, identified as a Phlyctochytrium sp. was isolated from G. fasciculatus sporocarps and propagated in sterile pond water on Gigaspora margarita spores or pollen of a Liquidambar sp. Parasitism by this fungus was nearly eliminated by addition of ethazole to the culture water.