The cultivated mushroom Agaricus bisporus was grown and fruited in wheat straw compost. RNA was extracted from samples of mycelium growing in compost during colonization and fruit body development. Laccase gene (combined lcc1 and lcc2) expression and cel3 gene (cellobiohydrolase) expression were determined by Northern blot analysis and by competitive RT-PCR. The level of laccase transcripts was highest in colonized compost (prior to the onset of fruiting), declined during the period of fruit body enlargement and increased again after harvesting and during the second flush of fruit body production. No transcripts of the cel3 gene were detectable in colonized or pre-fruiting cultures, but the level became detectable and rose to a maximum at the veil-break stage of fruiting, declined after harvesting and rose again during the second flush. Gene expression for laccase and cellobiohydrolase correlated with the known changes of laccase and cellulase enzyme activities during the fruiting life cycle.