p-Cresol at 17 mg l(-1) in a nitrifying culture inhibited by 70% nitrate formation whereas at 10 mg l(-1) there was no effect. p-Cresol at 220, 470, and 9 10 mg l(-1) was converted to intermediates after adaptation times of 8 h, 24 h, and 40 h, respectively. The sludge recovered 44% of its activity after transformation of p-cresol.