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Rifamycin-resistant derivatives of plant growth promoting Bacillus polymyxa strains L6, Pw-2, and S20 were used to evaluate the interaction of bacterial–mycorrhizal co-inoculation on pine and spruce seedling growth. We were particularly interested in determining if the mechanism by which bacteria stimulated seedling growth depended on the presence of ectomycorrhizae. Mycorrhizal inoculum was introduced by adding 2ml of one of six forest floor soil types originating from different spruce and pine stands to seedling containers. Mycorrhizal roots developed in 34% of pine and 27% of spruce seedlings treated with forest soil, but no differences between forest soils were detected. Most mycorrhizae were formed by Wilcoxina sp. (E-strain) (98% for spruce and 67% for pine); small numbers of Amphinema -like, Mycelium radicis atrovirens, Suillus -like …