The purpose of this work was to demonstrate that a Fenton (H(2)O(2)/Fe) reaction was involved in DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane)] degradation in a culture of Penicillium sp. spiked with FeSO(4). A commercial DDT mixture (10% DDE [1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene], 30% o,p-DDT and 60% of p,p' -DDT) of 10 mg L(-1) was used. Hydrogen peroxide (H(2)O(2)), tartaric acid and oxalic acid were identified at 18 h in culture media, with and without added DDT; this correlated positively with lowering of pH from 5.8 to 2.7. Lower concentrations of oxalic acid and H(2)O(2) (7.9 and 52.6 mg L(-1), respectively) occurred in media with DDT at 30 h, in comparison to that one without DDT mixture (27.9 and 65.3 mg L(-1), respectively), at this time there was maximum degradation (87.7, 91.7 and 94.2%) for DDE, o,p-DDT and p,p'-DDT, respectively. We propose that the degradation of the DDT mixture by Penicillium sp. was through a Fenton reaction (H(2)O(2)/Fe) under acidic conditions produced in situ during the fungal culture amended with FeSO(4).