Great Boiling Spring is a large, circumneutral, geothermal spring in the US Great Basin. Twelve samples were collected from water and four different sediment sites on four different dates. Microbial community composition and diversity were assessed by PCR amplification of a portion of the small subunit rRNA gene using a universal primer set followed by pyrosequencing of the V8 region. Analysis of 164 178 quality-filtered pyrotags clearly distinguished sediment and water microbial communities. Water communities were extremely uneven and dominated by the bacterium Thermocrinis. Sediment microbial communities grouped according to temperature and sampling location, with a strong, negative, linear relationship between temperature and richness at all taxonomic levels. Two sediment locations, Site A (87-80 degrees C) and Site B (79 degrees C), were predominantly composed of single phylotypes of the bacterial lineage GAL35 ([pmacr]=36.1%), Aeropyrum ([pmacr]=16.6%), the archaeal lineage pSL4 ([pmacr]=15.9%), the archaeal lineage NAG1 ([pmacr]=10.6%) and Thermocrinis ([pmacr]=7.6%). The ammonia-oxidizing archaeon ‘Candidatus Nitrosocaldus’ was relatively abundant in all sediment samples <82 degrees C ([pmacr]=9.51%), delineating the upper temperature limit for chemolithotrophic ammonia oxidation in this spring. This study underscores the distinctness of water and sediment communities in GBS and the importance of temperature in driving microbial diversity, composition and, ultimately, the functioning of biogeochemical cycles.