The first-order loss rate constant of methyl bromide was measured in surface seawater samples from the North Atlantic Ocean, Caribbean Sea, and coastal eastern Pacific Ocean during May--July 1998. A stable isotope incubation technique was used, which consisted of spiking seawater samples with 13CH3Br and following the loss rate of the isotopically labeled spike with gas chromatography and isotope dilution mass spectrometry. The analysis of both filtered and unfiltered aliquots of seawater provides insight into the relative importance of chemical and particle-related (presumably biological) degradation pathways. Over the entire cruise, first-order total degradation rate constants ranged from 0.03--0.40 day-1. On average, higher rate constants were observed during the last part of the cruise in the Caribbean Sea (mean 0.34 day-1) and coastal Pacific waters (0.31 day-1), than in the earlier passage through the North Atlantic Ocean (0.18 day-1, legs 1 and 3). In the warm waters of the Caribbean Sea and the Pacific, total degradation rate constants were controlled primarily by chemical losses (0.30 and 0.25 day-1, respectively). In the colder Atlantic waters the average chemical loss rate constant was lower (0.11 day-1), and biological losses constituted a significant fraction of the total loss rate constant (35%). Chemical loss rate constants varied strongly with water temperature, in good agreement with previous determinations of the rate constant for methyl bromide removal from seawater due to chloride substitution and hydrolysis. Biological losses were detected in most of the water samples analyzed, suggesting that the biological capability to degrade methyl bromide is ubiquitous in the oceans. Rate constants for biological removal exhibited no apparent relationship to seawater temperature, salinity, chlorophyll, or bacterial counts. ¿ 2001 American Geophysical Union