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With glucose as the carbon source, the ectomycorrhizal basidiomycete Hebeloma crustuliniforme (Bull. ex. Amans) Quel. utilizes nitrate, ammonium and amino acids as sources of nitrogen. Inoculated as a suspension of fine hyphae in such liquid media, the fungus expands rapidly, exhibiting a distinct exponential phase with a doubling time of 1–2 d and a maximum content of mycelial protein during the first 7 d of growth. Its growth when amino acids serve as sole carbon and nitrogen source is, however, less rapid, and requires several days of induction under these conditions. In H. crustuliniforme, assimilation of ammonium appears to be carried out primarily via the NADPH‐glutamate dehydrogenase (GDH) pathway. Its NADPH‐GDH levels in exponentially growing mycelia are several times greater than the apparent nitrogen assimilation rates of the mycelia. Its levels of glutamine synthetase activity in comparison …