Recent successful isolations (Dubinina et al., 1993) have been made from H2S or thiosulfate-rich samples on a medium containing (g/l): (NH 4) SO4, 1.0; MgSO4 7H30, 1.0; casein hydrolysate, 3.0; sodium succinate, 1.0; vitamins and trace elements(Pfennig and Lippert, 1966). The pH is 7.0, and the necessary redox gradient is generated by adding a FeS suspension, which also serves as a source of sulfide.