The 13C isotopic composition of methane emitted to the troposphere from Amazon capims (floating grassmats) ranged from -36.9 to -48.0 0/00, averaging -44.4¿4.2 0/00. All pools of methane associated with the grassmats were 13C enriched; methane withdrawn from plant stems ranged from -39 to -49 0/00 while bubbles stirred from the root mat averaged -41.4 0/00.As the CH4 flux from these habitats makes up some 40% of the total flux from the Amazon floodplain, CH4 emissions from the region as a whole must be enriched in the heavy carbon isotope. Methane withdrawn from the stems of five general of rooted macrophytes ranging in latitude from the Florida Everglades to the Alaskan Arctic exhibited 13C enrichment relative to the sedimentary methane bubble reservoir. Several hypotheses based upon isotopic fractionation by methane transport, oxidation and production processes are proposed to explain this phenomenon. ¿ American Geophysical Union 1989