Conditions to improve the efficiency of nitrogen removal were examined with the heterotrophic nitrifying/denitrifying bacterium Alcaligenes faecalis OKK17. This bacterium nitrifies ammonium and glutamate mainly to hydroxylamine and to a lesser extent to nitrite and nitrate, but it does not denitrify aerobically. Thus, the bacterium was immobilized in polyvinyl alcohol (PVA) hydrogel to provide anaerobic conditions in the gel support. Production of nitrous oxide in an oxygen-purged vial indicates that the nitrification products formed in the surface layer were denitrified in the subsurface layer of the gel. The specific total nitrogen removal activity by the immobilized cells in continuous how culture was higher than the specific nitrification activity by a cell suspension. A possible explanation of this acceleration of nitrification is that it is due to reduction of deleterious nitrification products, i.e., hydroxylamine and nitrite at mM concentrations, by subsequent denitrification.