Ten microfungal isolates from the rhizosphere of mycorrhiza of black spruce were tested for their ability to inhibit formation of mycorrhiza by Laccaria bicolor (Maire) Orton on axenically cultivated black spruce seedlings. Two Trichoderma species, T. viride Pers. and T. polysporum (Link ex Pers.) Rifai, were strongly antagonistic towards mycorrhizal colonization. This antagonism was seen both in trials where Trichoderma and L. bicolor were inoculated together and in a trial designed to allow establishment of the mycorrhizal fungus in the rhizosphere prior to a challenge with T. viride. Despite its antagonism in flask trials, T. viride did not parasitize the hyphae of L. bicolor in agar culture. Two other soil fungi, Tolypocladium inflatum Gams and Trichosoporon beigelii (Küch et Rab.) Vuill., also had small but significant inhibitory effects on the formation of mycorrhiza in flask trials. However, a variety of Mucoralean fungi, heavily sporulating soil Hyphomycetes and non-sporulating root-associated Hyphomycetes had no deleterious effect on the process. It is concluded that Trichoderma spp. are likely to be significant antagonists of mycorrhizal colonization in vivo, particularly in fumigated soils.