Laccase from the ascomycete Neurospora crassa is an inducible secretory enzyme. Production of this enzyme is repressed in vegetative cultures but can be induced by treatment with low concentrations of cycloheximide. Isolation and characterization of a derepressed mutant, the lah-l mutant, that is capable of producing laccase in vegetative cultures without induction by cycloheximide are described. The lah-l mutation is mapped between nit-2 and leu-3 on linkage group I, and it behaved as a recessive mutation in a forced heterokaryon. No differences were detected biochemically or immunologically between the laccase protein produced by the lah-1 mutant in the absence of cycloheximide and that induced with cycloheximide in the wild-type strain. This suggests that both laccases (66 kilodaltons) are products of the same structural gene. Relative amounts of laccase in the culture filtrate of the lah-l mutant were much higher than those induced with cycloheximide in the wild-type strain, demonstrating high efficiency of the lah-l mutant in production and secretion of laccase. The time course of laccase production by the lah-l mutant revealed that expression of 66-kilodalton laccase was repressed in conidia and derepressed during vegetative mycelial growth. This suggests that a multiple regulatory mechanism is involved in the production and/or maturation ofNeurospora laccase. The lah-l mutant may be useful for identifying genes that regulate expression of the laccase gene in N. crassa.