Water column depth profiles along the North Pacific margin from Point Conception to the tip of Baja California indicate elevation of nitrate (NO3-) 15N/14N and 18O/16O associated with denitrification in the oxygen-deficient thermocline waters of the eastern tropical North Pacific. The increase in δ18O is up to 3? greater than in δ15N, whereas our experiments with denitrifier cultures in seawater medium indicate a 1:1 increase in NO3- δ18O and δ15N during NO3- consumption. Moreover, the maximum in NO3- δ18O is somewhat shallower than the maximum in NO3- δ15N. These two observations can be summarized as an "anomaly" from the 1:1 δ18O-to-δ15N relationship expected from culture results. Comparison among stations and with other data indicates that this anomaly is generated locally. The anomaly has two plausible interpretations: (1) the addition of low-δ15N NO3- to the shallow thermocline by the remineralization of newly fixed nitrogen, or (2) active cycling between NO3- and NO2- (coupled NO3- reduction and NO2- oxidation) in the suboxic zone.