The distribution and availability of the nutrients nitrate and phosphate exert a strong control over primary production in the world ocean. Here we use a recently compiled global data set of oceanographic observations(1)-a database that is at least 40 times larger than currently used data sets (for example, ref. 2)-to analyse the variation of the nitrate : phosphate concentration ratio with depth and geographical location. Although the nutrient distributions confirm the dominant influence of aerobic decomposition, in agreement with the observations of Redfield(3), we also identify a hitherto unreported secondary trend at low nitrate : phosphate ratio (similar to 2-3). These conditions of low nitrate : phosphate ratio are associated with low oxygen concentrations and are probably caused by denitrification. Examination of the geographical distribution of these low nitrate:phosphate data suggests that denitrification in the western and northern North Pacific Ocean may have been previously overlooked, and that a reassessment of the global oceanic denitrification budget may therefore be required.