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Since little is known about the nitrogen (N) physiology of ectomycorrhizal fungi, this study was initiated to study both radial and biomass growth and N uptake of Hebeloma crustuliniforme and other Pacific Northwest ectomycorrhizal fungi. Hebeloma crustuliniforme utilized either nitrate or ammonium as the N source in buffered liquid media, but biomass growth was greatest with ammonium. Without buffers, biomass growth on ammonium-based media was reduced, apparently owing to low pH. These results emphasize the strong interrelationship between N source and pH of the media. A pronounced pH optimum for biomass growth of H. crustuliniforme occurred at pH 5.0. Ammonium uptake rates were five to nine times greater than nitrate uptake rates over a range of N concentrations (40–600 μM). Radial growth rates of a range of mycorrhizal fungi were greater on high-N than on low-N media. For those few species that grew faster on low-N medium, their growth habit was more diffuse and dry weight production was decreased, indicating that radial growth had occurred at the expense of biomass production.