We used13C-labeled methane to document the extent of trace methane oxidation byArchaeoglobus fulgidus,Archaeoglobus lithotrophicus,Archaeoglobus profundus,Methanobacterium thermoautotrophicum,Methanosarcina barkeriandMethanosarcina acetivorans. The results indicate trace methane oxidation during growth varied among different species and among methanogen cultures grown on different substrates. The extent of trace methane oxidation byMb. thermoautotrophicum(0.05 ± 0.04%, ± 2 standard deviations of the methane produced during growth) was less than that byM. barkeri(0.15 ± 0.04%), grown under similar conditions with H2and CO2.Methanosarcina acetivoransoxidized more methane during growth on trimethylamine (0.36 ± 0.05%) than during growth on methanol (0.07 ± 0.03%). This may indicate that, inM. acetivorans, either a methyltransferase related to growth on trimethylamine plays a role in methane oxidation, or that methanol is an intermediate of methane oxidation. Addition of possible electron acceptors (O2, NO3–, SO22–, SO32–) or H2to the headspace did not substantially enhance or diminish methane oxidation inM. acetivoranscultures.Separate growth experiments with FAD and NAD+showed that inclusion of these electron carriers also did not enhance methane oxidation. Our results suggest trace methane oxidized during methanogenesis cannot be coupled to the reduction of these electron acceptors in pure cultures, and that the mechanism by which methane is oxidized in methanogens is independent of H2concentration. In contrast to the methanogens, species of the sulfate-reducing genusArchaeoglobusdid not significantly oxidize methane during growth (oxidizing 0.003 ± 0.01% of the methane provided toA. fulgidus, 0.002 ± 0.009% toA. lithotrophicusand 0.003 ± 0.02% toA. profundus). Lack of observable methane oxidation in the threeArchaeoglobusspecies examined may indicate that methyl-coenzyme M reductase, which is not present in this genus, is required for the anaerobic oxidation of methane, consistent with the “reverse methanogenesis” hypothesis.