Methane fluxes were measured at locations across the Denver-Julesburg (DJ) basin of northeastern Colorado, which is in a semiarid climate. There is a potential for microseepage of thermogenically derived methane and light hydrocarbons to the atmosphere, which during drier or colder seasons may exceed the capacity for methanotrophic oxidation in the shallow soils. Triplicate measurements of methane flux in three seasons across the basin found areas of positive and negative fluxes in a patchy pattern which averaged +0.57 mg CH4 m-2 d-1. The possibility of drier areas of the continents, which are underlain by thick sedimentary units seasonally producing positive methane fluxes to the atmosphere, represents a second-order effect in the estimation of the terrestrial sources/sinks for atmospheric methane. ¿ 1998 American Geophysical Union