Nitrogen (N) is an essential nutrient in the sea and its distribution is controlled by microorganisms. Within the N cycle, nitrite (NO2(-)) has a central role because its intermediate redox state allows both oxidation and reduction, and so it may be used by several coupled and/or competing microbial processes. In the upper water column and oxygen minimum zone (OMZ) of the eastern tropical North Pacific Ocean (ETNP), we investigated aerobic NO2(-) oxidation, and its relationship to ammonia (NH3) oxidation, using rate measurements, quantification of NO2(-)-oxidizing bacteria via quantitative PCR (QPCR), and pyrosequencing. (15)NO2(-) oxidation rates typically exhibited two subsurface maxima at six stations sampled: one located below the euphotic zone and beneath NH3 oxidation rate maxima, and another within the OMZ. (15)NO2(-) oxidation rates were highest where dissolved oxygen concentrations were <5 μM, where NO2(-) accumulated, and when nitrate (NO3(-)) reductase genes were expressed; they are likely sustained by NO3(-) reduction at these depths. QPCR and pyrosequencing data were strongly correlated (r(2)=0.79), and indicated that Nitrospina bacteria numbered up to 9.25% of bacterial communities. Different Nitrospina groups were distributed across different depth ranges, suggesting significant ecological diversity within Nitrospina as a whole. Across the data set, (15)NO2(-) oxidation rates were decoupled from (15)NH4(+) oxidation rates, but correlated with Nitrospina (r(2)=0.246, P<0.05) and NO2(-) concentrations (r(2)=0.276, P<0.05). Our findings suggest that Nitrospina have a quantitatively important role in NO2(-) oxidation and N cycling in the ETNP, and provide new insight into their ecology and interactions with other N-cycling processes in this biogeochemically important region of the ocean.