procaryotes
   Domain Archaea Woese et al. 1990
     Phylum Euryarchaeota Garrity and Holt 2001
       Class Thermococci Zillig and Reysenbach 2001 see Class Thermococci Zillig and Reysenbach 2002
         Order Thermococcales Zillig 1988
           Family Thermococcaceae Zillig 1988
             1 Genus Thermococcus Zillig 1983
             2 Genus Pyrococcus Fiala and Stetter 1986