The nitrifying bacteria in activated sludge and biofilms consisting of the bacteria immobilized on polypropylene packing were subjected to an electric current via two electrodes. In activated sludge, the metabolism of nitrifying bacteria was inhibited when the applied current was over 2.5 A m(-2), whilst in biofilms, inhibition began when the current reached 5 A m(-2). At 15 A m(-2), the nitrification rate of NH4+-N in a biofilm with a bacterial density of 1.62 g total solids, dry wt m(-2) decreased to about 80% of its initial value. Ninety-two % of the initial biomass on the packing was retained after 36 h.