Measurements of the mixing ratio and Δ13C in methane (Δ13CH4) are reported from large, clean air samples collected every 2.5¿ to 5¿ of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for Δ13CH4 was highly dependent on season and varied from 0.5? in November 1996 with an estimated annual mean of 0.2--0.3?. The seasonal cycles in Δ13CH4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January--February for the extratropical Southern Hemisphere, in September--October for the tropics, and in June--July for the extratropical Northern Hemisphere. The data are compared with results from a three-dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either Δ13CH4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source-sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal Δ13CH4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH4 sink of 12--15?, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH+CH4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed. ¿ 1999 American Geophysical Union