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 temperature marshes (~-48%0 to ~-54%0), landfills (~-51%0 to ~-55%0), and the first reported values for any species of termite (-72.8¿3.1%0 for Reticulitermes tibialis and -57.3¿1.6%0 for Zootermopsis angustiacollis). Numbers in parentheses are Δ13C values with respect to PDB (Peedee belemnite) carbonate. Most methane sources reported thus far are depleted in 13C/12C ratios characteristic of methanisms 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%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 atmosphere CH4.