Forest soils are an important but under-studied part of forest ecosystems. The effects of O 3 on below-ground processes in a mature forest have only received limited attention so far. In our study, we have analysed the community of ectomycorrhizal fungi and beech fine root dynamics over two growing seasons (2003-2004) in a 70-year old mixed spruce-beech forest stand, in which two groups of five adult beech trees were either fumigated by 2× ambient ozone concentration or used as control. The main difference between previous studies and our approach was that previous studies were performed on seedlings in pot experiments or in closed or open top chambers, and not in situ, in a mature forest stand. Although beech is a relatively unresponsive species to tropospheric O 3, we found a pronounced effect of 2× O 3 on the number of vital ectomycorrhizal root tips and non-turgescent fine roots. Both categories of roots were significantly increased when compared to controls in two consecutive years at each sampling event. The number of types of ectomycorrhizae and species richness increased in 2004, but not in the extremely dry year 2003. We hypothesised that the observed changes might be an expression of a transitional state in below-ground succession of niches caused by an O 3 induced effect on carbon allocation to roots and the rhizosphere. We have detected changes in ectomycorrhizal species level, however Shannon-Weavers species diversity index and percentage of types of ectomycorrhizae did not change significantly in any sampling year thus indicating our results cannot be unequivocally explained by summer drought in year 2003 or by O 3 exposure alone.