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%o 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-δ 15N 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. Copyright 2005 by the American Geophysical Union.