The ability of the white rot basidiomycete Phanerochaete chrysosporium to transform s-triazine herbicides has been investigated in laboratory experiments. The chlorinated metabolites formed during atrazine N-dealkylations were not further transformed by the fungus, whereas hydroxyatrazine was converted to an unknown product. P. chrysosporium was also able to carry out the N-dealkylation of the herbicides simazine, propazine and terbuthylazine. Herbicide metabolism was not supported by purified peroxidases. The highest rates of herbicide N-dealkylation were obtained in liquid cultures maintained under moderate temperature allowing a long mycelium growing phase. Atrazine transformation was found to be supported by the mycelium, which contained significant amounts of microsomal cytochrome P450. Herbicide N-dealkylation was decreased in the presence of 1-aminobenzotriazole, in agreement with the involvement of P450 monooxygenases in atrazine metabolism.