Two species of novel, chemolithoautotrophic, sulfidogenic micro-organisms were isolated from submarine hydrothermal systems in the Atlantic (at the Kolbeinsey Ridge north of Iceland) and in the Pacific (at 9 degrees N, 104 degrees W). The coccoid cells grew within a temperature range of 70-98 degrees C (optimum around 90 degrees C). They gained energy by reduction of elemental sulfur using molecular hydrogen as the electron donor. 165 rDNA-based sequence comparisons revealed that the organisms are members of the crenarchaeal branch of the Archaea. They represent a new, deeply branching lineage within the family of the Desulfurococcaceae. In DNA-DNA hybridization experiments both strains exhibited low levels of hybridization to each other and to further representatives of this family. Therefore, they represent a new genus, for which the name Ignicoccus gen. nov. is proposed. At present it consists of two new species, Ignicoccus islandicus sp. nov. (type strain is Kol8T = DSM 13165T = ATCC 700957T) and Ignicoccus pacificus sp. nov. (type strain is LPC33T = DSM 13166T = ATCC 700958T).