Experiments were carried out to determine the mass balance of selenium biologically processed by a Se-resistant bacterium, Pseudomonas fluorescens K27. The bioprocessing of this toxicant, added to growing liquid cultures, resulted in reduction and in some part biomethylation that yielded Se species distributed between the solid, liquid, and gas phases. Initial anaerobic experiments with different amended selenium concentrations were carried out and the Se mass balance was determined using inductively coupled plasma spectrometry (ICP). Finally, bioreactor experiments were carried out under sequential anaerobic/aerobic culture growth conditions to determine the effects of these conditions on the production of elemental (solid) Se. As a percentage of amended selenium, selenite was more effectively reduced to metallic Se by R fluorescens than selenate under the conditions studied. When low amounts of selenite were present in the solution, reducing efficiency as a percentage of added Se was higher as measured by the production of Se-0. Sequential anaerobic/aerobic growth conditions did not have a significant effect in increasing the production of elemental selenium as carried out under these conditions: Yields of SO for anaerobic, sequential anaerobic/aerobic and microaerobic conditions were all approximately 36% of added Se in 1 mM selenite-amended cultures. (C) 2003 Elsevier Ltd. All rights reserved.