Chemotactic bacteria can be attracted to electron donors they consume. In systems where donor is heterogeneously distributed, chemotaxis can lead to enhanced removal of donor relative to that achieved in the absence of chemotaxis. However, simultaneous consumption of an electron acceptor may result in the formation of an acceptor gradient to which the bacteria also respond, thus diminishing the positive effect of chemotaxis. Depletion of an electron acceptor can also reduce the rate of electron donor consumption in addition to its effect on chemotaxis. In this study,we examined the effect of oxygen on chemotaxis to naphthalene and on naphthalene consumption by Pseudomonas putida G7. The organism was able to move up an oxygen gradient when there was a naphthalene gradient in the opposite direction. In the absence of an oxygen gradient, low levels of oxygen attenuated chemotaxis to naphthalene but did not affect random motility. The rate of naphthalene consumption decreased at dissolved oxygen concentrations similar to those at which chemotaxis was attenuated. These results suggest that low dissolved oxygen concentrations can reduce naphthalene removal by P. putida G7 in systems where naphthalene is heterogeneously distributed by simultaneously attenuating chemotactic motion toward naphthalene and decreasing the rate of naphthalene degradation. (c) 2005 Wiley Periodicals, Inc.