The effect of soil microorganisms on mineralization of C-14-labelled pyrene by white-rot fungi in solid-state fermentation was investigated. Two strains of white-rot fungi, Dichomitus squalens and a Pleurotus sp. were tested. The fungi were incubated on milled wheat straw contaminated with [C-14]pyrene for 15 weeks. CO2 and (CO2)-C-14 liberated from the cultures were determined weekly. To study the effect of soil microorganisms on respiration and [C-14]pyrene mineralization in different periods of fungal development, the fungal substrate was covered with soil at different times of incubation (after 0, 1, 3, 5, 7, 9 or 11 weeks). The two fungi showed contrasting ecological behaviour in competition with the soil microflora. Pleurotus sp. was highly resistant to microbial attack and had the ability to penetrate the soil. D. squalens was less competitive and did not colonize the soil. The resistance of the fungus was dependent on the duration of fungal preincubation. Mineralization of [C-14]pyrene by mixed cultures of D. squalens and soil microorganisms was higher than by the fungus or the soil microflora alone when soil was added after 3 weeks of incubation or later. With Pleurotus sp., the mineralization of [C-14]pyrene was enhanced by the soil microflora irrespective of the time of soil application. With D. squalens, which in pure culture mineralized less than did Pleurotus sp., the increase of [C-14]pyrene mineralization caused by soil application was higher than with Pleurotus sp.