The response of a mixed microbial culture to different feed compositions, that is, containing benzoate and pyruvate as sole carbon sources at different levels, was studied in a chemostat with a 48-h hydraulic residence time under cyclic aerobic and anoxic (denitrifying) conditions. The cyclic bacterial culture was well adapted to different feed compositions as evidenced by the lack of accumulation of benzoate or pyruvate in the chemostat. Both the benzoate-degrading capabilities and the in vitro catechol 2,3-dioxygenase (C23DO) activities of the cyclic bacterial cultures were in direct proportion to the flux through the chemostat of the substrate degraded by the pathway containing C23DO, with some exceptions. The quantity of C23DO showed a transient decrease during the initial portion of the aerobic period before returning to the level present during the anoxic period. That decrease was most likely caused by the production of H2O2 by the cells upon being returned to aerobic conditions. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier B.V. All rights reserved.