Thermophilic and extremely thermophilic enrichments from several different environments produced cell-associated emulsifiers as did several pure cultures ofArchaea. The bioemulsifiers were effective over a wide range of pH, at NaCl concentrations up to 200 g L−1, and at temperatures up to 80°C. The emulsifying activity in cell-free extracts ofMethanobacterium thermoautotrophicum was a cell-associated protein with a molecular weight greater than 5000 Da. This emulsifier formed viscous emulsions, but did not reduce the surface tension of water or the interfacial tension between water and hexadecane. The emulsifier had the greatest activity with alkanes with carbon numbers greater than 10. The characteristics of the bioemulsifier fromM. thermoautotrophicum makes it suitable for use in saline or thermophilic oil reservoirs as a mobility control agent or in well-bore clean up processes.