The isotopic composition (or18O/16O ratio) of O2produced by photosynthesis and the isotopic composition of O2consumed during respiration are different from each other as well as from the isotopic composition of ambient dissolved O2. In a closed body of water in which photosynthesis and respiration are co‐occurring, the18O/16O of dissolved O2will be a function of both the rate of respiration and the gross rate of photosynthesis. Therefore gross primary production can be calculated if one determines both net O2production and the18O/16O of dissolved O2. As a test of the feasibility of this approach, we present data on the18O/16O ratio of dissolved O2, along with ancillary parameters, in the top 200 m of waters in the North Pacific gyre in August and September 1985. The results are shown to reflect isotope fractionation during photosynthesis and respiration. We outline the additional information needed before natural18O/16O variations can be used to constrain quantitatively the gross productio