The carbon isotopic composition of methane emitted by the Alaskan emergent aquatic plants Arctophila fulva, a tundra mid-lake macrophyte, and Carex rostrata a tundra lake margin macrophyte, was -58.6¿0.5 (n=2) and -66.6¿2.5 (n=6)% respectively. The methane emitted by these species was found to be depleted in 13C by 12% and 18%, relative to methane withdrawn from plant stems 1 to 2 cm below the waterline. As the macrophyte-mediated methane flux represented approximately 97% of the flux from these sites, these results suggest the more rapid transport of 12CH4 relative to 13CH4 through plants to the atmosphere. This preferential release of the light isotope of methane, possibly combined with CH4 oxidation, caused by the buildup of the heavy isotope within plant stems. Plant stem methane concentrations ranged from 0.2 to 4.0% (x¿ 1.4; standard deviation (sd), 0.9 n=28) in Arctophila, with an isotopic composition of -46.1¿4.3% (n=8). Carex stem methane concentrations were lower, ranging from 150 to 1200 ppm (x¿, 500; standard deviation, 360; n=8) with an isotopic composition of -48.3¿1.4% (n=3). Comparisons of the observed isotopic fractionations with those predicted from gas phase effusion and diffusion coefficients suggest a combination of one or both of these gas transport mechanisms with bulk (non-fractionationating) flow. ¿American Geophysical Union 1992