We investigated the pathways of N 2 production in the oxygen-deficient water column of the eastern tropical South Pacific off Iquique, Chile, at 20uS, through short anoxic incubations with 15 N-labelled nitrogen compounds. The location was characterized by steep chemical gradients, with oxygen decreasing to below detection at ,50-m depth, while nitrite reached 6 mmol L 21 and ammonium was less than 50 nmol L 21 . Ammonium was oxidized to N 2 with no lag phase during the incubations, and when only NH þ 4 was 15 N-labeled, 15 N appeared in the form of 14 N 15 N, whereas 15 N 15 N was not detected. Likewise, nitrite was reduced to N 2 at rates similar to the rates of ammonium oxidation, and when only NO 2 2 was 15 N-labeled, 15 N appeared mainly as 14 N 15 N, whereas 15 N 15 N appeared in only one incubation. These observations indicate that ammonium was oxidized and nitrite was reduced through the anammox reaction, whereas denitrification was generally not detected and, therefore, was a minor sink for nitrite. Anammox rates were highest, up to 0.7 nmol N 2 L 21 h 21 , just below the oxycline, whereas rates were undetectable, ,0.2 nmol N 2 L 21 h 21 , deeper in the oxygen-deficient zone. Instead of complete denitrification to N 2 , oxidation of organic matter during the incubations may have been coupled to reduction of nitrate to nitrite. This process was evident from strong increases in nitrite concentrations toward the end of the incubations. The results point to anammox as an active process in the major open-ocean oxygen-deficient zones, which are generally recognized as important sites of denitrification. Still, denitrification remains the simplest explanation for most of the nitrogen deficiency in these zones. Anammox is the microbially catalyzed anaerobic oxida-tion of ammonium coupled to nitrite reduction, with production of nitrogen gas (Mulder 1989; van de Graaf et al. 1995), as indicated by the reaction