The carpophores of mycorrhizal fungi have declined drastically during this century in the Netherlands and in other European countries. In contrast, saprophytic and pathogenic fungi did not show a significant change. In this thesis, the possible causes of the decline of mycorrhizal mycoflora have been examined. The hypothesis was put forward that the functioning of mycorrhiza was hampered, either through a decrease of tree vitality or by changes in soil chemistry, both resulting from air pollution. Pinus sylvestris was chosen as study object, because in the Netherlands (1) its vitality has decreased considerably, (2) mycorrhizal mycoflora of coniferous tree species decreased more strongly than that of deciduous species, (3) it is the only native conifer which possesses ectomycorrhizas and (4) plantations of P. sylvestris of the same age and on the same soil type can be found throughout the country.In a pot experiment, mycorrhizas of Paxillus involutus appeared to be sensitive to SO 2 fumigation alone, or in combination with NH 3 pollution, in contrast to mycorrhizas of Laccaria proxima . Photosynthesis, measured on P. involutus -inoculated seedlings, was inhibited by SO 2 fumigation. However, effects on plant growth were negligible (Chapter 2).Nitrate and ammonium salts in a pot experiment had a significant negative effect on the mycorrhizas ( Paxillus involutus and Suillus bovinus ), and a significant positive effect on plant growth. Ammonium treatments affected the seedlings more positively and the mycorrhizas more negatively than nitrate. The N content of seedling needles fertilized with ammonium was higher than those treated with nitrate. It was suggested that a high N uptake by the plant decreased the carbohydrate availability for the mycorrhizal fungi (Chapter 3).Ammonium and nitrate fertilization at rates of 0, 30 and 60 kg N ha -1yr -1in two young stands of P. sylvestris during three years had a similar, significantly negative effect on the number and total dry weight of carpophores and on the number of ...