The results of Δ13C measurements of several types of major sources of methane are as follows: rice paddies, -67%; the peat bogs of the Lake Agassiz region of northern Minnesota, -67¿5%; swamps of the Florida Everglades, -55¿3% and biomass burning, -24 to -32%. In addition, results are presented of a study of the Δ13C of CH4 released from a slough, compared to the CH4 in the bottom sediment. These isotopic values are used, together with previously published data, to make up a tentative budget of the fluxes of the major sources for atmospheric methane with an averge isotopic composition matching the measured value for atmospheric CH4, taking into account the fractionation effect of the sink processes. This budget requires the existence of a significant flux from an anthropogenic source of heavy CH4, calculated to be 45¿15 Tg yr-1 if attributed to CH4 from biomass burning, with ΔC=-25%. ¿ American Geophysical Union 1988