A detailed immunologic analysis of Methanothrix soehngenii Opfikon (the type species of the genus), Methanothrix sp. strain CALS-1, and Methanothrix concilii GP6 was performed. A variety of poly- and monoclonal antibody probes for a comprehensive panel of reference organisms were used to determine immunogenicity, antigenicity, and relationships. The three organisms are antigenically distinct but interrelated, forming an immunologically cohesive group, weakly related to methanosarcinae. A prominent feature of the organisms was heterogeneity characterized by antigenic diversity and compartmentation, the latter particularly evident in M. soehngenii Opfikon, which was examined more thoroughly. The complexity of the antigenic profile of this strain and the heterogeneity of the group suggest a high degree of phenotypic diversification within the genus.