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Ratios of 13C/12C have been measured in atmospheric methane and in methane collected from sites and biota that represent potentially large sources of atmospheric methane. These include temperate marshes (∼−48‰ to ∼−54‰), landfills (∼−51‰ to ∼−55‰), and the first reported values for any species of termite (−72.8 ± 3.1‰ for Reticulitermes tibialis and −57.3 ± 1.6‰ for Zootermopsis angusticollis). Numbers in parentheses are δ13C values with respect to PDB (Peedee belemnite) carbonate. Most methane sources reported thus far are depleted in 13C with respect to atmospheric methane (−47.0 ± 0.3‰). Individual sources of methane should have 13C/12C ratios characteristic of mechanisms of CH4 formation and consumption prior to release to the atmosphere. The mass-weighted average isotopic composition of all sources should equal the mean 13C of atmospheric methane, corrected for a kinetic isotope effect in the OH attack of CH4. Assuming the kinetic isotope effect to be small (∼−3.0‰ correction to −47.0), as in the literature, the new values given here for termite methane do not help to explain the apparent discrepancy between 13C/12C ratios of the known CH4 sources and that of atmospheric CH4.