Anammox, the oxidation of ammonium with nitrite to dinitrogen gas under anoxic conditions, is an important process in mesophilic environments such as wastewaters, oceans and freshwater systems, but little is known of this process at elevated temperatures. In this study, we investigated anammox in microbial mats and sediments obtained from several hot springs in California and Nevada, using geochemical and molecular microbiological methods. Anammox bacteria-specific ladderane core lipids with concentrations ranging between 0.3 and 52 ng g(-1) sediment were detected in five hot springs analyzed with temperatures up to 65 degrees C. In addition, 16S rRNA gene analysis showed the presence of genes phylogenetically related to the known anammox bacteria Candidatus Brocadia fulgida, Candidatus Brocadia anammoxidans and Candidatus Kuenenia stuttgartiensis (96.5-99.8% sequence identity) in three hot springs with temperatures up to 52 degrees C. Our data indicate that anammox bacteria may be able to thrive at thermophilic temperatures and thus may play a significant role in the nitrogen cycle of hot spring environments.