Mycorrhizas have been shown to alter ion fluxes for many nutrients. Experiments were done to determine the extent to which ectomycorrhizas alter the ionic balance of roots of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings mycorrhizal with Hebeloma crustuliniforme (Bull, ex St. Amans) Qúel. exposed to solutions containing either 16N ammonium sulphate or 16N ammonium chloride. Net fluxes of ammonium, hydrogen, potassium, calcium, sodium, bicarbonate, sulphate and chloride were measured. The mycorrhizal association altered ion fluxes, primarily decreasing sodium uptake and potassium release rates (both anion treatments) and increasing bicarbonate release during exposure to ammonium chloride. Rates of carboxylate accumulation in the tissue were inferred from the net flux data; mycorrhizal seedlings may produce more cation-carboxylate complexes compared with nonmycorrhizal seedlings. When compared with agricultural plants exposed to ammonium, Douglas-fir took up a considerably greater number of cations for each ammonium ion absorbed, which may account for the greater number of carboxylate anions also produced by Douglas-fir. These findings were discussed in terms of ecological significance for growth of Douglas-fir.