Atmospheric methyl bromide is of considerable environmental importance as the largest reservoir of gaseous bromine in the atmosphere. Bromine gases can catalytically destroy stratospheric ozone. Since agricultural activities, automobiles, biomass burning, and other human activities produce CH3Br, it is of interest to know its global mass balance and particularly the specific source and sinks. In this paper we provide a decadal time series of global CH3Br concentrations in the Earth's atmosphere. The data show that average concentrations are about 10 pptv and during the last 4 years may be increasing at 0.3¿0.1 pptv/yr (3%/yr¿1%/yr). We estimate that the atmospheric lifetime of CH3Br that is due to reaction with OH, is about 2 years, resulting in a calculated global emission rate of about 100 Gg/yr. Ocean supersaturations of 140--180% are observed, and atmospheric concentrations over the open oceans are higher than at comparably located coastal sites. The ocean source is estimated to be about 35 Gg/yr. The remaining emissions must come from other natural sources and anthropogenic activities. Our results are based on some 2200 samples obtained over more than a decade. Mass balance calculations explain most aspects of the present data but other implications are not easily reconciled, leaving open the possibility of undiscovered sources and sinks. ¿ American Geophysical Union 1993