The phenotypic and genotypic properties of a new coccoid hyperthermophilic archaeum (3-5 mu m in diameter) which has been isolated from a hot alkaline spring near Hveragerdi, Iceland, are described. The new strain is a strict anaerobe and grows fermentatively on yeast extract. Growth is inhibited by elemental sulfur. Under laboratory conditions, growth is observed between 70 and 95 degrees C at pH 6.5 to 8.5. Its membrane lipids consist exclusively of dibiphytanyldiglycerol tetraethers. Respiratory lipoquinones are absent. The G+C content of the DNA has been determined to be 54.7 ± 0.8 mol%. On the basis of its 16S rDNA sequence the strain is affiliated to the crenarchaeotal branch of Archaea. The next relatives are Desulfurococcus mobilis (4.5% dissimilarity) and Pyrodictium occultum (7.0% dissimilarity). Due to the phenotypic and genotypic properties of this new isolate which distinguish it from other currently known Archaea, we propose to placing it in a new genus, Sulfophobococcus with the first species S. zilligii in honour of WOLFRAM ZILLIG, a pioneer in the field of hyperthermophilic Archaea.