A three-dimensional ocean-atmosphere model of the global methyl bromide budget is presented including an analysis of the temporally and spatially varying source and sink distributions. No correlation is found between oceanic net biological CH3Br production, implied by surface water saturation anomaly observations, and oceanic variables such as surface chlorophyll and primary production; therefore model ocean fluxes are constrained directly by the saturation anomaly observations. The resulting diagnosed biological production rates imply net production in the tropics and subtropics and net consumption at high latitudes. Results from this semicoupled ocean-atmosphere model show substantial longitudinal variability in the atmospheric boundary layer CH3Br concentrations with land-ocean contrasts of 1--6 ppt due to regional industrial and agricultural emissions on land and net fluxes into the ocean. Owing to an imbalance in current understanding of the global budget, our simulated mixing ratios of 3.5 to 6.5 ppt for the southern and northern hemispheres, respectively, are significantly lower than available measurements. Sensitivity studies reducing the ocean and soil surface sinks slightly improve the global mean CH3Br concentration but increase the interhemispheric ratio further beyond that supported by observations. Accordingly, an apparent terrestrial missing source of 89--104 kT yr-1 is derived and applied to the model. This is of the same order as the sum of all other sources in the model (85 kT yr-1). The hemispheric distribution of the missing source is explored, indicating that 50--70% of this source occurs in the southern hemisphere and is likely to be biased toward tropical regions. Modeled seasonal variability in the interhemispheric ratio at specific monitoring sites agrees well with observations. The model-predicted vertical gradient of CH3Br through the troposphere and lower stratosphere is also presented. ¿ 1998 American Geophysical Union