To explore the spatial variation of the soil microbial community within a mixed Norway spruce-birch stand, and to test if the spatial patterns of the microbial community are related to the position of trees, we sampled the forest floor at two spatial scales and used the phospholipid fatty acid (PLFA) patterns as indicators of the microbial community structure. Of the 32 most common PLFAs, 20 (62%) were clearly spatially autocorrelated, and the limit of spatial dependence (range) varied between 1 m and 11 m. The variation in the community structure was examined by subjecting the PLFAs to a principal component analysis. The first two principal components described variation structured at two different spatial scales. The range of the microbial community for the first component was 4.6 m, whereas for the second component it was only 1.5 m. The microbial community was influenced by the position of the trees. Spruce trees had a much stronger influence on PLFA patterns than birch trees, and the first principal component, as well as 12 PLFAs, was influenced by spruce trees. Several branched PLFAs, characteristic of Gram-positive bacteria, loaded negatively on the second principal component. These PLFAs represent a complex of associated microorganisms that aggregated in small patches away from birch trees. A comparison with a laboratory experiment suggests that although the tree species differ in their influence on soil moisture and ground vegetation, their influence on the microbial community were, to a large extent, connected to the quality of soil organic matter associated with the two trees. (C) 2000 Elsevier Science Ltd. All rights reserved.