ABSTRACTThe tall fescue endophyte,Acremonium coenophialum, is inhibitory to mycorrhizal fungi. This study was conducted to determine ifAcremoniuminhibits infection and the extent of colonization of roots in addition to sporulation. Fewer fescue seedlings, with or withoutAcremonium, were infected by single spores ofGlomus macrocarpumthan pearl millet seedlings. However, fescue seedlings were infected byG. mosseaesingle spores to the same extent as pearl millet seedlings. Roots ofAcremonium-free fescue were not colonized as extensively as those of pearl millet by either fungus. The presence ofAcremoniumin fescue caused a further considerable reduction in extent of mycorrhizal colonization. Effects of the host on sporulation were closely correlated with extent of colonization. Each of the three hosts supported a similar total spore volume of the two mycorrhizal fungi.Acremoniumapparently does not affect infection, perhaps because the seedlings were too young for extensive transport of toxic alkaloids fromAcremonium-infected shoot tissues to roots free ofAcremonium.