The toxicity of a sublethal cadmium concentration to ectomycorrhizal Pinus sylvestris L. seedlings was studied. Nine mycorrhizal strains, collected from a Zn- and Cd-polluted soil or from an unpolluted area, were compared for their ability to increase Cd tolerance in their host plant. Plants were cultivated in a semi-hydroponic system that allowed visual observation of the root systems. The degree of soil colonization as well as the density of the extramatrical mycelium was determined. The water use of the host plants was carefully monitored during the treatment period. Cadmium concentrations were determined in all plant parts. Although Cd addition resulted in decreases in transpiration in the non-mycorrhizal plants, disturbances in water relations could be more or less pronounced in the mycorrhizal pines. The treatment strongly influenced the growth of the fungi in the substrate: several mycobionts hardly survived in the polluted substrate. Fungi producing a large mycelial biomass showed the greatest effect in overcoming toxicity. Some s trains were even able to increase their turnover growth when treated with cadmium. In a dense extramatrical mycelium the potential for Cd retention increases, while the individual hyphae are exposed to a lower Cd concentration than in a sparse mycelium. A protective effect against cadmium toxicity in the host was observed with all mycobionts, since Cd uptake was highest in the non-mycorrhizal seedlings. The treatment had no effect on the growth of the seedlings or on the concentrations of the other cations, probably due to the relatively low toxicity and the relatively short observation period. The results are discussed in relation to the in vitro Cd tolerance of the fungi.