In an in vitro study, interactions between ectomycorrhizal fungi, saprotrophic basidiomycetes and soil microfungi known from pine forests were examined in relation to different ammonium concentrations. The saprotrophic basidiomycetes Clitocybe marginella and C. vibecina and the soil microfungi Penicillium pinetorum, Oidiodendron flavum and Torulomyces lagena were grown against the ectomycorrhizal Cenococcum geophilum, Laccaria bicolor and Rhizopogon luteolus. Interactions were species specific and overgrowth was the most common (43%), followed by inhibition at distance (26%) and contact inhibition (16%). Ammonium concentrations affected the type and strength of the interactions. The competitive strength of the ectomycorrhizal fungi was relatively high and only the growth of the mycelia of L. bicolor was clearly suppressed by the Clitocybe spp. The interactions are discussed in relation to the ecology of the fungi and potential future studies.