The degradation rate constant of CH3Br in the North Pacific Ocean was measured in surface seawater between September and October 1999, using a stable isotope (13CH3Br) incubation technique. Total degradation rate constants ranged from 0.02 to 0.43 d-1, decreasing in colder waters as a result of the temperature-dependence of chemical losses. Biological rate constants ranged from 0.01 to 0.20 d-1. In subtropical waters (13--20¿N), biological loss rate constants were small compared to chemical loss rate constants. North of Hawaii, biological processes played an increasingly significant role in CH3Br degradation. In subpolar waters (40--58¿N), biological losses dominated the removal of methyl bromide. Comparison of the measured loss rate constants with surface water CH3Br concentrations suggest that the CH3Br production rate is higher in warm, low latitude waters than in cold subpolar waters at this time of year. Diel studies revealed a midday maximum in biological degradation of methyl bromide. ¿ 2001 American Geophysical Union