Methane flux from plots ofPeltandra virginicain a bottomland hardwood swamp in southeast Virginia varied from 270 to 670 mg CH4m−2d−1from May to August and was 6 times greater than the flux from unvegetated adjacent open waters. Variations in chamber temperature, light, and CO2levels failed to produce systematic changes in CH4flux rate or its isotopic composition from vegetated plots, suggesting that CH4flux fromPeltandrais independent of stomatal aperture or hourly variations in photosynthetic rate. Comparisons of different chamber techniques for the collection of CH4for isotopic analysis and for flux measurements fromPeltandrasuggest that the effect of using simple uncontrolled chambers is small relative to temporal and spatial variations in methane emission and its isotopic composition. These results may be extended to other aquatic macrophytes which also transport gas primarily by molecular diffusion such asOryza, Carex, Pontederia, Sagittaria, and Cladium.The δ13C of CH4released fromPeltandravaried between −61 and −71 °/oo. Isotopic variations in CH4associated withPeltandraappeared to be controlled by transport effects. Methane withdrawn fromPeltandrastems was considerably13C enriched relative to sedimentary CH4and was not an indicator of the isotopic composition of CH4emitted by these plants. Hence emergent aquatic macrophytes rooted in organic‐rich sediments do not appear to be a source of13C enriched methane to the atmosphere. Emitted CH4was13C depleted relative both to the CH4within stems (−49 to −55 °/oo) and within sediments (‐56 to ‐58 °/oo), suggesting isotopic fractionation associated with the release of CH4by the plants. The preferential release of12CH4by the plants is partly compensated for by the enrichment of13C in CH4contained within the plant stem. Although the presence of methanotrophic bacteria within the rhizosphere ofPeltandrawas demonstrated, the stable isotopic composition of methane emitted from these plants or collected from sedimentary bubbles and plant stems presents no evidence for extensive oxidation of methane withinPeltandraor its rhizosphere. Either stable isotopes are not a good indicator of rhizopheric methane oxidation or the microbes' in situ respiration is limited by competition for oxygen with the demands of root respiration, oxidation of complex organic matter, ferrous iron oxidati