We report δD and δ13C measurements of atmospheric CH4 from air samples collected from two locations in the United States. They are the mid continental site Niwot Ridge, Colorado (400N, 1050W), and a Pacific coastal site receiving strong westerlies, Montaqa de Oro, California (350N, 1210W). Data from multiyear approximately bimonthly sampling provide information relating seasonal cycling of CH4 sources and sinks in background air, record long-term trends in CH4 mixing and isotope ratio related to the atmospheric CH4 loading, and may indicate regional CH4 sources. At Niwot Ridge, δD-CH4 averaged -93.1 1 3.0? from 1999 to 2001, while δ13C-CH4 averaged -47.22 1 0.13? from 1995 to 2001 with distinct seasonal cycles in both isotope ratios. At Montaqa de Oro, atmospheric CH4 was observed to be more depleted in 13C and D: Measured δD-CH4 averaged -97.3 1 3.7? from 2000 to 2001, while δ13C-CH4 averaged -47.26 1 0.17? from 1996 to 2001, and seasonal cycles were larger than those observed at Niwot Ridge. Mixing ratios observed at Montaqa de Oro were higher on average than at Niwot Ridge. At both sites, δ13C-CH4 was found to correlate poorly with mixing ratio, an indication that varying CH4 sources are partly responsible for the δ13C-CH4 seasonal signal. In contrast, a strong anticorrelation exists between δD-CH4 and mixing ratio, with maxima and minima approximately 6 months out of phase, indicating a sensitivity of δD to sink processes. The dual isotopic constraint to atmospheric CH4 seasonality implies that these midlatitude sites are annually influenced by a 13C-enriched CH4 source(s) seasonally increasing in late spring and a 13C-depleted CH4 source(s) seasonally increasing in late summer or early fall.