The carbon kinetic isotope effect in the CH4 + HO reaction can enrich atmospheric methane in 13C relative to its sources. In the only previous measurement of the carbon kinetic isotope effect in this reaction, a value of k12/k13 of 1.003 was reported. In the present experimental determination we have found a significantly larger kinetic isotope effect of 1.010 ¿ 0.007 (95% confidence interval). This report discusses the experimental details of the carbon kinetic isotope effect determination in the CH4 + HO reaction and the possible atmospheric implications in the use of the present result. Tropospheric methane is known to have a 13C/12C ratio of -47% (parts per thousand) relative to Peedee belemnite, and most known methane sources for which measurements of the stable carbon isotope ratio have been performed are depleted in 13C relative to the atmosphere. A k12/k13 ratio of 1.010 leads to an expected value for the weighted average of the 13C/12C ratio of the methane sources which is about 10% more depleted than the atmosphere (approximately -57%). This result is fundamentally important to those researchers who use stable carbon isotope ratios to study atmospheric methane. ¿American Geophysical Union 1987