A chemostat containing a mixed microbial community was operated with a hydraulic retention time of 48 h under cyclic aerobic/anoxic conditions while receiving a feed containing benzoate and pyruvate as electron donors. The cycle time was 12 h, with a 6-h aerobic period and a 6-h anoxic period. Oxygen was provided as the terminal electron acceptor (TEA) during the aerobic period, and nitrate was provided during the anoxic period. Both were supplied in excess. Some of the time the denitrifying bacteria in the chemostat left the residual anoxic TEA in the form of nitrate, but some of the time they left it in the form of nitrite. The aerobic benzoate biodegradative capability (AeBBC) of the culture was much greater when nitrite was present than when nitrate was present, as were the levels of catechol 1,2-dioxygenase and catechol 2,3-dioxygenase in the cells. However, the culture exhibited similar anoxic BBCs in the presence of nitrate and nitrite, even though the level of benozyl-CoA reductase was higher in the presence of nitrite. Neither the aerobic nor the anoxic pyruvate biodegradative capability was greatly affected by the presence or absence of nitrite. Analysis of the microbial community structure by denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplified rDNA revealed changes in that structure, but none that could be associated with the presence or absence of nitrite.