A bacterial strain, AN3, which was able to use aniline or acetanilide as sole carbon, nitrogen and energy sources was isolated from activated sludge and identified as Delftiasp. AN3. This strain was capable of growing on concentrations of aniline up to 53.8 mM (5000 mg/l). Substituted anilines such as N-methylaniline, N, N-dimethylaniline, 2-methylaniline, 4-methylaniline, 2-chloroaniline, 3-chloroaniline, o-aminoaniline, m-aminoaniline, p-aminoaniline, and sulfanilic acid did not support the growth of strain AN3. The optimal temperature and pH for growth and degradation of aniline were 30 degrees C and 7.0, respectively. The activities of aniline dioxygenase, catechol 2,3-dioxygenase and other enzymes involved in aniline degradation were determined, and results indicated that all of them were inducible. The K (m) and V (max) of aniline dioxygenase were 0.29 mM and 0.043 mmol/mg protein/min, respectively. The K (m) and V (max) of catechol 2, 3-dioxygenase for catechol were 0.016 mM and 0.015 mmol/mg protein/min, respectively. Based on the results obtained, a pathway for the degradation of aniline by Delftiasp. AN3 was proposed. The importance of the strain to the operation of municipal wastewater treatment plants is discussed.