Using primers designed on the basis of sequence homologies in the copper-binding domains for a number of plant and fungal tyrosinases, two tyrosinase encoding cDNAs were cloned from an Agaricus bisporus U1 cDNA-library. The sequences AbPPO1 and AbPPO2 were, respectively, 1.9 and 1.8 kb in size and encoded proteins of approximately 64 kDa. The cDNAs represent different loci. Both AbPPO1 and AbPPO2 occur as single copies on the genomes of the U1 parental strains H39 and H97. The genomic size of AbPPO1 and AbPPO2 is minimally 2.3 and 2.2 kb, respectively. Alignment and phylogenetic analysis of 35 tyrosinase and polyphenol oxidase sequences of animal, plant, fungal, and bacterial origin indicated conserved copper-binding domains, and stronger conservation within genera than between them. The translation products of AbPPO1 and AbPPO2 possess putative N-glycosylation and phosphorylation sites and are recognised by antibodies directed against a 43-kDa tyrosinase. The observations are consistent with previously proposed maturation and activation models for plant and fungal tyrosinases.