Thermophilic (45 degrees to 75 degrees C) bacteria that reduce amorphous Fe(III)-oxyhydroxide to magnetic iron oxides have been discovered in two geologically and hydrologically isolated Cretaceous-and Triassic-age sedimentary basins in the deep (>860 meters below land surface) terrestrial subsurface. Molecular analyses based on 16S ribosomal RNA (rRNA) gene sequences revealed that some of these bacteria represent an unrecognized phylogenetic group of dissimilatory Fe(III)-reducing bacteria. This discovery adds another dimension to the study of microbial Fe(III) reduction and biogenic magnetism. It also provides examples for understanding the history of Fe(III)-reducing microorganisms and for assessing possible roles of such microorganisms on hot primitive planets.