Abstract: The condition and types of ectomycorrhizae of Scots pine Pinus sylvestris and the biological activity of soil micro-organisms in the humus layer of polluted pine forests were studied in the surroundings of Oulu, northern Finland. Biological activity, expressed as the rate of humus respiration and as dehydrogenase enzyme activity, decreased towards the central, most polluted area with higher sulphur and nitrogen contents in the humus layer. The ectomycorrhizae of Scots pine were less ramified and poorly developed in the central area. The number of mycorrhizal types and the number of mycorrhizal species encountered in the sporophore inventories were lower in the two most polluted zones than in the cleaner sites. The type composition of the mycorrhizae was altered in the polluted sites.