To allow a full understanding of the global methane cycle, the contribution made by microbes as a sink for methane must be investigated. Many methane-oxidizing bacteria present in environmental samples are not easily cultured by using normal microbiological techniques, leading to an underestimate of the numbers of such bacteria in the marine and terrestrial environment. We have used DNA sequence information from cloned genes coding for the methane-oxidizing system in bacteria to develop highly specific methane-gene probes in order to detect methane oxidizing bacteria in environmental samples without the need for laboratory cultivation of these organisms. Considerable enhancement of detection by these probes has been achieved using the polymerase chain reaction technique.