The capacity of Serratia marcescens culture, both in planktonic cultures and in activated carbon biofilm, to reduce chromium(VI) to insoluble chromium(III) was evaluated at different times by capillary zone electrophoresis (CZE). The simultaneous determination of both chromium species by CZE made evident the reduction of Cr(VI) to Cr(III) showing a decrease in the concentration of Cr(VI) and an increase in the formation of Cr(III) in a single run. When comparing this activity between planktonic and sessile cultures, the latter showed a higher reductive activity, removing 89% of Cr(VI) present in the media after 48 h . On the other hand, only 80% of Cr(VI) was reduced to Cr(III) by Ser. marcescens free cells of cultures during a similar period