Neurospora crassa possesses an inducible L-amino acid oxidase that is expressed only when cells are derepressed for nitrogen in the presence of an amino acid. Enzyme synthesis requires both induction by an amino acid and simultaneous nitrogen catabolite derepression. Carbon limition in the presence of an amino acid does not permit induction of L-amino acid oxidase. The nit-2 gene is a major regulatory locus which is believed to mediate nitrogen catabolite repression in Neurospora. Mutants of nit-2 are repressed for L-amino acid oxidase activity under conditions which lead to good enzyme induction in wild type and nit-2 revertants. The loss of the enzyme in nit-2 mutants does not result from inducer exclusion, which suggests that the nit-2 gene product has a direct role in controlling the expression of this enzyme. Substantial amounts of L-amino acid oxidase were detected in the growth medium as well as in cell extracts of the wild type strain. Biochemical data indicates that the intracellular and the extracellular L-amino acid oxidases are identical. Inhibitors of protein and of RNA synthesis block accumulation of L-amino acid oxidase, suggesting that enzyme expression is controlled at the level of transcription. D-amino acid oxidase can be detected in cell extracts of Neurospora grown in the presence of a D-amino acid. The enzyme is present in cys-3 mutants and is not repressed by high concentrations of sulfate or nitrogen indicating that D-amino acid oxidase is not a member of the sulfur or nitrogen regulatory circuits of this organism.