We examined rates of N 2 fixation from the surface to 2000 m depth in the Eastern Tropical South Pacific (ETSP) during El Niñ o (2010) and La Niñ a (2011). Replicated vertical profiles performed under oxygen-free conditions show that N 2 fixation takes place both in euphotic and aphotic waters, with rates reaching 155 to 509 mmol N m 22 d 21 in 2010 and 24614 to 118687 mmol N m 22 d 21 in 2011. In the aphotic layers, volumetric N 2 fixation rates were relatively low (,1.00 nmol N L 21 d 21), but when integrated over the whole aphotic layer, they accounted for 87-90% of total rates (euphotic+aphotic) for the two cruises. Phylogenetic studies performed in microcosms experiments confirm the presence of diazotrophs in the deep waters of the Oxygen Minimum Zone (OMZ), which were comprised of non-cyanobacterial diazotrophs affiliated with nifH clusters 1K (predominantly comprised of a-proteobacteria), 1G (predominantly comprised of c-proteobacteria), and 3 (sulfate reducing genera of the d-proteobacteria and Clostridium spp., Vibrio spp.). Organic and inorganic nutrient addition bioassays revealed that amino acids significantly stimulated N 2 fixation in the core of the OMZ at all stations tested and as did simple carbohydrates at stations located nearest the coast of Peru/Chile. The episodic supply of these substrates from upper layers are hypothesized to explain the observed variability of N 2 fixation in the ETSP.