Pseudomonas aeruginosa 142 was grown on benzoate as sole carbon Source in chemostats under fully aerobic condition and tested for the aerobic ability to degrade benzoate at maximum oxygen uptake rate (OUR) in a batch reactor after exposure to anaerobic conditions in the absence of substrate. Aerobically grown P. aeruginosa 142 lost around 75% initial max OUR in 8 h of anaerobic exposure. Laboratory experiments with aerobic ring-cleaving enzymes showed that a small part of the 75% loss of activity was due to the aerobic ring-cleaving enzymes (i.e. C12DO) while the remainder was due to cytochrome oxidases. (C) 2004 Elsevier Ltd. All rights reserved.