Although removal of tropospheric methyl bromide (CH3Br) is dominated by the reaction with the seasonally varying hydroxyl (HO) radical concentration, the anticipated corresponding seasonal dependence of CH3Br, as found for other gases with major HO sinks, has been sought previously without success [WMO, 1995>. Our observations of northern hemispheric boundary layer CH3Br concentrations do reveal substantial seasonal changes. The high latitude CH3Br North/South interhemispheric concentration ratio (IHR) varies from a maximum of 1.35¿0.04 (1&sgr;) in March--April to 1.10¿0.04 in September, with an equal area and seasonally (EAS) weighted average IHR of 1.21¿0.03. These observations suggest northern hemispheric emissions are about 15 kilotons/year less than when an IHR of 1.3 is considered [WMO, 1995>. The observed seasonality also partially explains the differences in the IHR reported by several research groups [WMO, 1995> and places needed constraints on the magnitude and seasonality of sources and sinks of CH3Br. ¿ 1998 American Geophysical Union