Aims: The aims of this study were (i) to develop a protocol for the entrapment of anaerobic (hyper) thermophilic marine micro-organisms; (ii) to test the use of the chosen polymers in a range of physical and chemical conditions and (iii) to validate the method with batch cultures. Methods and Results: The best conditions for immobilization were obtained at 80 degrees C with gellan and xanthan gums. After 5-week incubation, beads showed a good resistance to all tested conditions except those simultaneously including high temperature (100 degrees C), low NaCl (<0.5 mol l(-1)) and extreme pH (4/8). To confirm the method efficiency, batch cultures with immobilized Thermosipho sp. strain AT1272 and Thermococcus kodakarensis strain KOD1 showed an absence of detrimental effect on cell viability and a good growth within and outside the beads. Conclusion: This suggests that entrapment in a gellan-xanthan matrix could be employed for the culture of anaerobic (hyper) thermophilic marine microorganisms. Significance and Impact of the Study: (Hyper) thermophilic marine microorganisms possess a high biotechnological potential. Generally microbial cells are grown as free-cell cultures. The use of immobilized cells may offer several advantages such as protection against phage attack, high cell biomass and better production rate of desired metabolites.