ABSTRACT The effect of salinity and growth temperature on the accumulation of intracellular organic solutes was examined in the hyperthermophilic archaeon Palaeococcus ferrophilus . The genus Palaeococcus represents a deep-branching lineage of the order Thermococcales , which diverged before Thermococcus and Pyrococcus. Palaeococcus ferrophilus accumulated mannosylglycerate, glutamate, and aspartate as major compatible solutes. Unlike members of the genera Pyrococcus and Thermococcus , Palaeococcus ferrophilus did not accumulate di- myo -inositol phosphate, a canonical solute of hyperthermophiles. The level of mannosylglycerate increased in response to both heat and salt stress; glutamate increased at supraoptimal growth temperatures, whereas aspartate increased at supraoptimal salt concentration. Proline, alanine, and trehalose were also found in lesser amounts, but their levels did not respond significantly to any of the stresses imposed. Additionally, the genes involved in the synthesis of mannosylglycerate in Palaeococcus ferrophilus and Thermococcus litoralis were identified. In both organisms the synthesis proceeds via the two-step pathway comprising mannosyl-3-phosphoglycerate synthase (MPGS) (EC 2.4.1.217) and mannosyl-3-phosphoglycerate phosphatase (MPGP) (EC 3.1.3.70). The mpgS and mpgP genes of Palaeococcus ferrophilus were expressed in Escherichia coli and the proteins were characterized. MPGS had maximal activity at 90°C and pH near 7.0, and was strictly dependent on Mg ²⁺ . MPGP had optimal activity at 90°C and pH 6.0 and was barely dependent on Mg ²⁺ . The half-life values for inactivation of MPGS and MPGP at 83°C were 18 and 25 min, respectively. A comparative discussion of the osmo- and thermoadaptation strategies in these three genera of the Thermococcales is presented.