The endopolysaccharide accumulated by Thermococcus hydrothermalis was extracted and purified from a 4 h culture. It presented an "amylopectin-like" structure with an average chain length of 14 and a ramification degree of 7.5%. The glucosyltransferase was isolated, partially purified and characterized. The molecular mass was 42 k-Da by SDS PAGE and 85+/-5 k.Da by gel filtration. This enzyme was able to use both Uridine-5'-DiPhosphoGlucose (UDPG) and Adenosine-5'-DiPhosplioGlucose (ADPG) as substrates. Optimal pH and temperature for the enzyme were 5.5 and 80degreesC, respectively. In the presence of 3.2 mM ADPG, the half life of the protein was 6 min at 110degreesC. The apparent K-m value with the two substrates was 0.9 mM, but the V-max was 9.7 fold higher for ADPG. A branching activity was also detected at high temperature, up to 80degreesC by different methods: phosphorylase stimulation, iodine, and branching linkage assays.