The flux of particulate matter through the oceanic water column is a primary component in elemental cycling and is generally perceived as being in one direction: downward1, 2. The organic matter constituting these particles is produced through photosynthesis in surface waters and either sinks directly as phytoplankton and products3, 4 or undergoes various trophic transformations through the water column. A large proportion of the particulate organic matter produced in surface waters is regenerated in the euphotic zone5–7. A fraction of this organic matter, however, leaves the surface waters and settles through the water column, generally decreasing in quantity and changing in quality with increasing distance from the surface8–11. Although the net transport of organic matter must be downward to fuel the lower portions of the water column, there is also an upward component to transport. Positively buoyant particles, including lipid-rich eggs, larvae and, possibly, carcasses of deep-sea animals are examples of particles which undergo upward transport12–13. A previous attempt to quantify the upward mass flux indicated rates of 1–4% of the downward mass flux14. Here we report the first evidence that there is a significant upward flux of particulate organic matter, up to 66.7% of the concurrently measured downward flux, at two stations in the deep North Pacific. Given this magnitude, the previously ignored upward flux of such organic matter must be considered in models of carbon and nitrogen cycling in the open ocean.