Immobilized bacterial cell technology was applied, on a bench scale, to the selective removal of nitrate from contaminated water, together with the segregation of denitrifying bacteria and the carbon energy source from the treated water. The two-chambered reactor, with a microporous membrane for bacterial cell immobilization, performed at an average denitrification rate of 5,800 mg nitrate-nitrogen (NO3--N)/m2d of membrane surface area. A carbon-to-nitrogen mass consumption ratio of 2.2 g organic carbon/gram nitrogen was observed, with a small degree of methanol contamination of the water being treated. A physical model for NO3--N removal based on Fick's law and the measured diffusion coefficient of nitrate through the immobilization structure gave a good correlation with experimental results. Includes 39 references, table, figures.