Pseudomonas butanovora,, a novel denitrifying bacterium, was immobilized in composite beads and filled into a reactor, system. The pilot bioreactor average denitrification activity was at ethanol-C:nitrate-N ratios of 3:1 and 1.5:1 +/- 0.88 and 0.54 kg NO3--N m(-3) d(-1), respectively. The denitrification was stable in spite of the felatively low hydraulic retention times of 2.47 and 3 h. The nitrate content of the influent was almost completely reduced at the first level of the bioreactor and the nitrite formed underwent reduction in the upper part of the reactor. The experimentally determined optimum ethanol-C:nitrate-N ratio was 1.41 +/- 0.41. In consequence of the aerobic conditions, the acetic acid produced by the oxygenation of ethanol was also detectable in the reactor effluent. The pH of the effluent (7.58) never exceeded the acceptable maximum (8.5). The nitrate removal efficiency of the cells was nearly 100% at both C:N ratios, and the nitrite content of the effluent was around the prescribed limit throughout the continuous operation. This continuous-flow pilot bioreactor containing immobilized P. butanovora cells proved an efficient denitrification system with a relatively low retention time.