Several yellow-pigmented isolates, with optimum growth temperatures of about 65-70 degrees C, were recovered from hot springs in Iceland. Phylogenetic analysis of the 16S rDNA and DNA-DNA reassociation values showed that these organisms represented two new species of the genus Thermus. Strains RF-4T and HN1-8 had maximum temperatures for growth below 80 degrees C, while strains HN3-7T and HN2-7, unlike all other strains of the species of the genus Thermus except those belonging to Thermus thermophilus, grew at 80 degrees C. The new isolates from Iceland could not be distinguished easily from each other or from other strains of the species of the genus Thermus by biochemical characteristics; however, strains RF-4T and HN1-8 assimilated ribitol, a characteristic which was not detected in any of the other strains examined. Moreover, the species represented by strains RF-4T and HN1-8 and the species represented by strains HN3-7T and HN2-7 could be distinguished clearly from the other species of Thermus by their fatty acid composition. Strains RF-4T and HN1-8 have the highest combined levels of iso-15:0 and iso-17:0 and the lowest levels of iso-16:0 of any of the strains of the species of Thermus, while strains HN3-7T and HN2-7 are characterized by a very low iso-15:0/iso-17:0 ratio. On the basis of the phylogenetic analysis, DNA-DNA reassociation values, physiological and biochemical characteristics and fatty acid composition, the name Thermus igniterrae sp. nov. is proposed for the species represented by strains RF-4T and HN1-8 and the name Thermus antranikianii sp. nov. is proposed for the species represented by strains HN3-7T and HN2-7.