In order to improve the microbial reaction rate within a cell-immobilized space, this study experimentally investigated enhanced substrate transport by electrophoresis of a specific ion. Batch-mode and continuous feeding denitrification tests were carried out in an autotrophic denitrifying bioreactor using sulfur-oxidizing microbes as a model system. Seed sludge and powdered sulfur were immobilized on fibrous carrier medium, and an electric field was applied through the cell-immobilized zone via inert electrodes on both sides of vessel. In batch-mode denitrification tests, nitrogen removal rate reached 1.99 g-N (.) m(-3). h(-1) when the electric field of 18 V was reversed at intervals of 30 min, as compared with 0.40 g-N (.) m(-3) (.) h(-1) when the electric field was not applied. These findings demonstrated that the denitrification rate was increased by promoting the transport of NO3 ions across the cell-immobilized zone by electrophoresis. In addition, no reduction of NO3- to NO2- occurred, indicating that decrease of nitrogen compound was due to biological reaction. Mathematical simulation analysis using a model involving Monod-type reaction kinetics and the Stokes formula for NO3- mobility was in general agreement with the experimental results. The results of the continuous feeding test also indicated that electrophoresis improved the nitrogen removal rate.