The constitutive intracellular laccase activity of ten strains of Pleurotus spp. was determined in vitro and by zymograms, using different substrates. Differences in the in vitro activities were observed between all the strains; however, zymogram patterns were only similar for strains within same species, independently of any of the three substrate (2,6-dimethoxyphenol, p-anisidine or o-tolidine) used. The differences observed in the number and positions of the isoforms in the gel suggest that laccase zymograms can be used to differentiate species of this organism.