Despite the importance of nitrous oxide (N 2 O) in the global radiative balance and atmospheric ozone chemistry, its sources and sinks within the Earth's system are still poorly understood. In the ocean, N 2 O is produced by microbiological processes such as nitrification and partial denitrification, which account for about a third of global emissions. Conversely, complete denitrification (the dissimilative reduction of N 2 O to N 2) under suboxic/anoxic conditions is the only known pathway accountable for N 2 O consumption in the ocean. In this work, it is demonstrated that the biological assimilation of N 2 O could be a significant pathway capable of directly transforming this gas into particulate organic nitrogen (PON). N 2 O is shown to be biologically fixed within the subtropical and tropical waters of the eastern South Pacific Ocean, under a wide range of oceanographic conditions and at rates ranging from 2 pmol N L 21 d 2 to 14.8 nmol N L 21 d 21 (mean 6 SE of 0.52261.06 nmol N L 21 d 21 , n = 93). Additional assays revealed that cultured cyanobacterial strains of Trichodesmium (H-9 and IMS 101), and Crocosphaera (W-8501) have the capacity to directly fix N 2 O under laboratory conditions; suggesting that marine photoautotrophic diazotrophs could be using N 2 O as a substrate. This metabolic capacity however was absent in Synechococcus (RCC 1029). The findings presented here indicate that assimilative N 2 O fixation takes place under extreme environmental conditions (i.e., light, nutrient, oxygen) where both autotrophic (including cyanobacteria) and heterotrophic microbes appear to be involved. This process could provide a globally significant sink for atmospheric N 2 O which in turn affects the oceanic N 2 O inventory and may also represent a yet unexplored global oceanic source of fixed N.