The flux of biologically produced organic carbon from the euphotic zone of the ocean to the deep waters below-the ’biological organic carbon pump’-is one of the main controls on the carbon dioxide partial pressure in the atmosphere(1). Accurate determination of this flux is therefore critically important for understanding the global carbon cycle and its response to climate change. Our goal is to assess how accurately the biological organic carbon pump can be determined at a single location and to constrain estimates of the global value. As there are no standards against which such environmental fluxes can be measured, we assess accuracy by comparing results from three independent experimental approaches for measuring the net annual export of organic carbon from the euphotic zone in the subtropical North Pacific Ocean near Hawaii. Mass balances of dissolved oxygen, inorganic carbon and organic carbon yield estimates of the organic carbon export flux of 2.7 +/- 1.7, 1.6 +/- 0.9 and 2.0 +/- 0.9 mol C m(-2)yr(-1), respectively, These three estimates are not significantly different, and establish the present analytically attainable accuracy at this location to be about +/-50%. If 2.0 mol C m(-2)yr(-1) is typical of the organic carbon export flux in the subtropical ocean, then this vast region, often considered to be a biological desert, may be responsible for up to half of the global-ocean biological organic carbon pump.