methanococci methanococci methanococci is a class of methanogenic archaea in the phylum euryarchaeota. they can be mesophilic, thermophilic or hyperthermophilic. methanococci scientific classification domain: archaea kingdom: euryarchaeota phylum: euryarchaeota class: methanococci order methanococcales synonyms methanococci boone 2002 methanothermea methanothermea cavalier-smith 2002