This study examined the role of ectomycorrhizal associations in nitrogen assimilation of Populus tremuloides seedlings. Seedlings were inoculated with Hebeloma crustuliniforme and compared with non-inoculated plants. Nitrogen-metabolizing enzymatic properties were also determined in H. crustuliniforme grown in sterile culture. The seedlings and fungal cultures were subjected to nitrogen treatments (including NO₃⁻, NH₄⁺ and a combination of NO₃⁻ + NH₄⁺) for 2 months to examine the effects on growth, nitrogen-assimilating enzyme activities and xylem sap concentrations of NH₄⁺ and NO₃⁻. Seedlings were also provided for 3 days with ¹⁵N-labeled NH₄⁺ and NO₃⁻, and leaf and root ¹⁵N content relative to total nitrogen was measured. Both NO₃⁻ and NH₄⁺ were effective in supporting seedling growth when either form was provided separately. When NO₃⁻ and NH₄⁺ were provided together, seedling growth decreased while enzymatic assimilation of NH₄⁺ increased. Additionally, nitrogen assimilation in inoculated seedlings was less affected by the form of nitrogen compared with non-inoculated plants. Fungal ability to enzymatically respond to and assimilate NH₄⁺ combined with aspen's enzymatic responsiveness to NO₃⁻ was likely the reason for efficient assimilation of both nitrogen forms by mycorrhizal plants.