The 18O/16O of dissolved oxygen was measured in the upper ocean of the subarctic Pacific in 1988. In May and August, at stations Papa (50¿N, 145¿W) and R (53¿N, 145¿W), the mean Δ18O in the mixed layer was 23.84¿0.20 and 24.00¿0.24 O/OO (versus standard mean ocean water) consistently more depleted than atmospheric saturation levels by about 0.4 O/OO. This relative depletion is caused by input of photosynthetically produced O2. A value for the isotopic fractionation effect during respiration (αT) of 0.978¿0.006 was determined from the time rate of change of the concentration and Δ18O of O2 in in the mixed layer measured during August 1988. Below the mixed layer (100--280 m) the O2 concentration decreased with a corresponding increase in Δ18O. Model derived values for αT over this depth region ranged from 0.980 to 0.988 and depended on the mixing model. The difference between αT determinations for the surface layer versus upper thermocline likely results from mixing model inaccuracies or different isotope fractionation effects during plankton and bacterial respiration. If the calculated mixed layer αT values apply oceanwide, then photosynthesis and respiration by the marine biota have a similar effect to land plants in maintaining the Δ18O of atmospheric O2 at 23.5 O/OO. ¿ American Geophysical Union 1993