The hyperthermophilic archaeon Thermococcus hydrothermalis produces a reserve endopolysaccharide during its exponential growth phase. This endopolymer was purified from the crude intracellular content after a 3-hour and a 4-hour culture (corresponding to ¾ of the log phase and to the end of this phase respectively). After analysis of the endopolysaccharide it appeared that its structure changed with the physiological cellular states: A glycogen type structure (with 8% of -1,6 linkages) was purified from the 3 hours culture whereas an “amylopectin-like” structure (7,5% of -1,6 branching points and glucanic chains of 14 -1,4 linked glucose residues) was purified at the end of the log phase. These new results show that this former “amylopectin-like” is a very ephemeral structure, built prior to the polymer degradation. Moreover, the polymer biosynthesis involves one partially purified elongation activity and probably only one branching activity. The optimal pH and temperature for the elongation enzyme was observed at 80-85°C and at pH 5.5 respectively. The branching activity was optimally active between 70°C and 80°C and between pH 4 to pH 7. Together, the presence of a glycogen-like structure and the number of isoform involved in its biosynthesis suggest a relationship between Archaea and the other prokaryotic organisms.