A change in the nature of the residual terminal electron acceptor in an anoxic (denitrifying), carbon-limited, benzoate-degrading chemostat from nitrate to nitrite was taken as a sign of a change in the structure and/or function of the microbial community. The results from denaturing gradient gel electrophoresis showed that although the structures of the microbial communities in the chemostat were very similar under the two conditions, there were also differences. Biomass samples were collected from the chemostat during the nitrate residual and nitrite residual periods to see how they differed in terms of the stability and inducibility of their benzoate biodegradative capability (BBC). Biomass taken from the chemostat during the nitrate residual period almost completely lost its anoxic BBC within I h when exposed to aerobic conditions in a fed-batch reactor (FBR). In addition, following 16-h exposure to aerobic conditions, it did not recover that capability within 9 h after being returned to anoxic conditions. In contrast, biomass taken from the chemostat during the nitrite residual period retained at least 20% of its anoxic BBC after 9 h in the aerobic FBR and rapidly recovered that capability within 3 h upon being returned to anoxic conditions. Biomass taken from the chemostat during the nitrate residual period did not develop aerobic BBC within 9 h in an aerobic FBR; however, biomass taken from the chemostat during the nitrite residual period developed that ability within 6 h and then slowly lost it over a 6-h period in an anoxic FBR. Nitrite itself was not responsible for these different observations. Rather, they appeared to depend upon the nature of the bacteria in the communities.