The effect of ammonium chloride was tested on mycorrhiza formation in Picea abies inoculated with mixed inocula of 8 ectomycorrhizal fungi in the very early stage of root growth after germination. Weekly doses of 1.64 and 13.12 mg ammonium chloride pot^-1, equalling 50 and 400 kg N ha^-1 year^-1 showed no effect on infectivity and competitivity in mixture I, where the ammonium tolerant fungus Hebeloma crustuliniforme was dominant. In mixture II, where Laccaria laccata by the treatment. Growth of seedlings was significantly improved by ammonium fertilization in mixture I, while in mixture II this effect was much less pronounced. Practical consequences of the results are discussed.