Methyl fluoride (MF) and dimethyl ether (DME) were effective inhibitors of aerobic methanotrophy in a variety of soils. MF and DME blocked consumption of CH(4) as well as the oxidation of CH(4) to CO(2), but neither MF nor DME affected the oxidation of [C]methanol or [C]formate to CO(2). Cooxidation of ethane and propane by methane-oxidizing soils was also inhibited by MF. Nitrification (ammonia oxidation) in soils was inhibited by both MF and DME. Production of N(2)O via nitrification was inhibited by MF; however, MF did not affect N(2)O production associated with denitrification. Methanogenesis was partially inhibited by MF but not by DME. Methane oxidation was approximately 100-fold more sensitive to MF than was methanogenesis, indicating that an optimum concentration could be employed to selectively block methanotrophy. MF inhibited methane oxidation by cell suspensions of Methylococcus capsulatus; however, DME was a much less effective inhibitor.