The influence of ectomycorrhizal fungi on the soil bacterial community was studied by growing pine seedlings in artificial soils consisting of a peat/sand mixture amended with microcline, biotite or apatite. In the microcline-amended and unamended soils both Suillus variegatus and Paxillus involutus reduced bacterial activity as measured by thymidine incorporation. S. variegatus grew best in the biotite soil, where it increased both bacterial activity and biomass as measured by microscopic counts and specific bacterial fatty acids. Further, the positive influence of S. variegatus on the bacteria in the biotite soil modified the bacterial community, as reflected in the bacteria-specific phospholipid fatty acid composition. The increases in bacterial biomass and activity and changes in the bacterial community induced by S. variegatus may be due to the production of organic substances by this fungus, as indicated by an 10-fold increase in soil-solution citric acid. Two isolates of S. variegatus and an unidentified ectomycorrhizal fungus all tended to stimulate bacterial activity in the apatite-amended soil in compartments isolated from roots by a mesh. We conclude that the same ectomycorrhizal fungus may stimulate bacterial growth under certain conditions and inhibit bacterial growth under other conditions.