The total methane flux to the troposphere from tundra environments of the Yukon-Kuskokwim Delta is dominated by emissions from wet meadow tundra (∼75%) and small, organic-rich lakes (∼20%). The mean δ13C value of methane diffusing into collar-mounted flux chambers from wet meadow environments near Bethel, Alaska, was −65.82 ± 2.21‰ (±1 sigma, n = 18) for the period July 10 to August 10, 1988. Detritus-rich sediments of Delta lakes, including margins of large lakes and entire submerged areas of smaller ones, are laden with gas bubbles whose methane concentration ranges from 11% to 79%. Lowest methane concentrations are found along heavily vegetated lake edge environments and highest through-out organic-rich, fibrous sediments of small lakes. A minimum ebullition flux estimated for the 5% of total Delta area comprised of small lakes ranges from 0.34 to 9.7 × 1010 g CH4 yr−1, which represents 0.6% to 17% of the total Delta methane emission. The δ13C and δD values of this ebullitive flux are −61.41 ± 2.46‰ (n = 38) and −341.8 ± 18.2‰ (n = 21), respectively. The methane in gas bubbles from two lakes is of modern, bomb carbon enriched, radiocarbon age. Gas bubble δ13C values varied from 2 to 5‰ seasonally, reaching heaviest values in midsummer, no such variations in δD values were observed. Combined isotope data reveal that higher δ13C values in heavily vegetated areas correlate with lower δD values, suggesting enhanced methane production via acetate fermentation. Spatial isotopic variations in lakes appear to be controlled by variations in production rather than oxidation processes.