Pleurotus ostreatus is a white-rot fungus known as an efficient degrader of lignin and also various organo-pollutants. Using a DNA-mediated transformation system, a molecular breeding approach to isolate overproducers of a manganese peroxidase isozyme, MnP3, was carried out. Recombinant mnp3 constructs under the control of P. ostreatus us sdi1 expression signals were introduced into the wild-type P. ostreatus strain by co-transformation with a carboxin-resistant vector plasmid, pTM1. One of the recombinants obtained by a mating between two monokaryotic transformants, TMG9-C1, showed a several. times higher level of MnP activity than the wild-type control in the early stage of liquid culture. Predominant transcription of the recombinant mnp3 in the strain was demonstrated by RT-PCR experiments. This is the first report of a genetically modified P. ostreatus strain with an expression system for recombinant genes.