During the May -- June, 2000 Eastern Pacific Redox Experiment (EPREX) we examined the dynamics of methane (CH4) in the eastern tropical North Pacific (ETNP), a large region of high surface-ocean productivity fueled by coastal upwelling. We discovered that (1) the ETNP contains by far the largest pool of CH4 yet discovered in the open ocean; (2) CH4 production in the upper half of this subsurface pool is associated with the decomposition of locally produced sinking particulate matter; (3) the deeper half of this pool is from a coastal source; (4) advection and oxidation of the upper pool leads to the heavy CH4 isotopic values seen at midwater in the North Pacific subtropical gyre; and (5) the ETNP is a source of isotopically enriched CH4 to the atmosphere. Our results suggest that other oceanic areas of upwelling-induced anoxia may be sites of significant atmospheric input of isotopically heavy CH4. ¿ 2001 American Geophysical Union