METHANE is a greenhouse gas whose concentration in the atmosphere is increasing. Much of this methane is derived from the metabolism of methane-generating (methanogenic) bacteria and over the past two decades much has been learned about the ecology of methanogens; specific inhibitors of methanogenesis, such as 2-bromoethanesulphonic acid, have proved useful in this regard. In contrast, although much is known about the biochemistry of methane-oxidizing (methanotrophic) bacteria, ecological investigations have been hampered by the lack of an analogous specific inhibitor. Methanotrophs limit the flux of methane to the atmosphere from sediments and consume atmospheric methane, but the quantitative importance of methanotrophy in the global methane budget is not well known. Methylfluoride (CH3F) is known to inhibit oxygen consumption by Methylococcus capsu-latus, and to inhibit the oxidation of 14CH4 to 14CO2 by endosymbionts in mussel gill tissues. Here we report that methylfluoride (MF) inhibits the oxidation of methane by methane monooxy-genase, and by using methylfluoride in field investigations, we find that methanotrophic bacteria can consume more than 90% of the methane potentially available.