The origin of methane in selected groundwater flow systems is investigated through carbon isotope geochemical studies combined with hydrogeological and hydrogeochemical information. Methane is a common trace constituent in these groundwaters and occasionally constitutes a major carbon pool. Two modes of origin for methane are commonly recognized: microbial production (biogenic methane) and non-biological, chemical production (thermocatalytic methane). In the groundwaters sampled, the few thermocatalytic methanes encountered have δ13C values greater than about −45‰ whereas the more common normal biogenic methanes have δ13C values less than about −60‰. Biogenic methanes from sanitary landfill sites have intermediate δ13C values of about −38 to −56‰ and so can easily be distinguished from normal biogenic methane.The carbon source for biogenic methane is usually organic matter present in the aquifer materials. The carbon isotope ratios of dissolved methane and total inorganic carbon in sampled groundwaters are not controlled by equilibrium fractionation processes but by kinetic isotope effects. Secondary processes, such as microbial oxidation of methane, are important in establishing the carbon isotope ratio in some methanes.