The methane dynamics in the waters of Lakes Michigan and Superior, components of the North American Great Lake system, were investigated using measurements of methane concentration and natural radiocarbon (14C‐CH 4 ) dissolved in these lake waters. All 14C‐CH 4 measurements were above modern levels regardless of location and depth with a range of 117‐145% modern carbon (pMC). Methane concentrations in the deep basin of both lakes were low, ranging from 3.3 to 4.3 nM, with minimal vertical variation. However, the concentrations of CH 4 increased toward coastal areas in both lakes, possibly due to higher groundwater inputs and aerobic methanogenesis associated with primary productivity. Except for one site, 14C‐CH 4 dissolved in the waters of Lake Michigan was greater than in Lake Superior by ~12 pMC, a difference that was likely due to inputs of excess 14CH 4 from nuclear power plants along the coast of Lake Michigan.