Four laccase isozyme genes, Psc lac1, 2, 3 and 4 have been cloned from the edible mushroom, Pleurotus sajor-caju. The genes display a high degree of homology with other basidiomycete laccases (55-99%) at the amino acid level. Of the laccase genes isolated, Psc lac1 and 4 displayed the highest degree of similarity (85% at the amino acid level), while Psc lac3 showed the highest degree of divergence, exhibiting only 52-57% amino acid similarity to the other PL: sajor-caju laccase gene sequences. Laccase activity in PL: sajor-caju is affected by nutrient nitrogen and carbon, and by the addition of copper and manganese to the growth medium. In addition, 2,5-xylidine, ferulic acid, veratric acid and 1-hydroxybenzotriazole induced laccase activity in the fungus. Induction of individual laccase isozyme genes by carbon, nitrogen, copper, manganese and the two aromatic compounds, 2,5-xylidine and ferulic acid, occurred at the level of gene transcription. While Psc lac3 transcript levels appeared to be constitutively expressed, transcript levels for the other laccase isozyme genes, lac1, 2 and 4, were differentially regulated under the conditions tested.