procaryotes
   Domain Archaea Woese et al. 1990
     Phylum Euryarchaeota Garrity and Holt 2001
       Class Methanococci Boone 2001 nom. inval. see Class Methanococci Boone 2002
         Order Methanococcales Balch and Wolfe 1981
           Family Methanococcaceae Balch and Wolfe 1981
             1 Genus Methanococcus Kluyver and van Niel 1936 (Approved Lists 1980)
             2 Genus Methanothermococcus Whitman 2001 nom. inval. see Genus Methanothermococcus Whitman 2002