Continuous culture of P. aeruginosa was conducted with nitrate-containing media under the dilution rates (D) of 0.026, 0.06, and 0.13/h and the dissolved oxygen concentrations (DO) of 0-2.2 mg/L. The bacterium performed simultaneousO(2) and nitrate respiration in all of the systems studied. For each D, the (apparent) cell yield from glucose (Y-X/S) was lower at zero DO, but did not change substantially with non-zero DO. In non-zero DO systems, Y-X/S increased with increasing D, and when fit with a model considering cell death, gave the following parameters: maximum cell yield Y-X/S(m) = 0.49, maintenance coefficient M-S = 0.029 (/h), and cell decay constant k(d) = 0.014/h. The same model failed to describe the behaviors of zero-DO systems, where neither glucose nor nitrate was limiting and the limiting factor(s) remained unknown. The cell yield from accepted electron (Y-X/e) was however relatively constant in all systems, and the energy yield per electron accepted via denitrification was estimated at similar to 69% of that via O-2 respiration. A closer examination revealed that increasing DO enhanced O-2 respiration only at extremely low DO (<0.05 mg/L), beyond which the increasing DO only slightly increased its weak inhibition on denitrification. While O-2 was the preferred electron acceptor, the fraction of electrons accepted via denitrification increased with increasing D. (c) 2006 Wiley Periodicals, Inc.