Activated sludge obtained from a coke-plant wastewater treatment plant was acclimated and used as seeding microbes to investigate the kinetics of biodegradation of di-n-butyl phthalate (DBP), one of the most frequently identified phthalic acid esters in diverse environments and the most widely used plasticizer in China. DBP biodegradation conformed to the first-order model with respect to its concentration and the rate constant was independent of the initial concentration when the initial DBP concentration was less than 200 mg/litre.