However, there are some anomalies. Whilst the reorganization of the taxonomy of the methanogens as suggested by Boone et al. (1993) is broadly supported by our data, some of the more specific taxonomic relationships are not confirmed. An example is the position of Methanosphaera stadtmanae, which is considered to be a member of the order Methanobacteriales, grouping within this order by 16S rRNA analysis, but not by mcrA analysis (Fig. 2). Methanosphaera stadtmanae is an unusual member of the Methanobacteriales, having a coccoid shape, unlike all the rest which are rods, and growth is limited to the reduction of methanol by H2. The mcrA sequence we obtained is more closely related to the mrtA sequences of other Methanobacteriales (Fig. 3). It is therefore possible that either our primers failed to amplify the Methanosphaera stadtmanae mcrA gene fragment, or it is no longer present, as we were only able to detect this mrtA-like sequence. The rest of the Methanobacteriales fall readily into the taxonomic organization proposed by Boone et al. (1993) and confirmed by Wasserfallen et al. (2000). Similarly, the reorganization of the Methanococcales into two families and four genera is supported.