The effect of ammonium on a denitrifying reactor of the upflow anaerobic sludge blanket type was studied. At a constant nitrate loading rate (2500 mg NO3 --N/[L. d]), using acetate as organic electron donor and at a C/NO3 --N/ ratio of 1.23, an increase in the N2 production rate was observed when the ammonium loading rate was increased (25, 250, and 500 mg NH4 +-N/[L. d]). Dissimilatory nitrate reduction to ammonium (DNRA) was not observed, and the N2 production efficiency was increased from 84 to 100% or higher. Since NH4 + in the output was lower than in the input, it was suggested that it was used for nitrate reduction. At constant NH4 +-N/NO3 --N/ and C/NO3 --N/ ratios of 0.2 and 1.63, respectively, the molecular nitrogen production rate was increased at 300 and 500 mg NH4 +-N/(L. d), whereas at 200 mg NH4 +- N/(L. d) DNRA took place probably owing to culture conditions of low reductive power. Molecular nitrogen production was not observed under autotrophic conditions, and the addition of acetate to the culture recovered its high nitrogen removal rate. Experimental results and balances indicated that the consumed ammonium was used as an additional reductive source.