Two isolates of a novel, moderately thermophilic Thiobacillus species have been studied. The isolates, KU and BC13, are Gram-negative, motile bacteria having a pH optimum for growth of 2-2.5 and an optimum growth temperature of 45 degrees C. Both isolates are capable of chemolithotrophic growth on reduced sulfur substrates. They can also use molecular hydrogen as an electron donor. These two isolates can grow mixotrophically with sulfur or tetrathionate and yeast extract or glucose. The G+C content is 63.1-63.9 mol% and the isolates exhibit no significant DNA homology to any other Thiobacillus species. Strains KU and BC13 both contain ubiquinone Q-8. 16S rRNA analysis indicates that these strains belong to a group of bacteria which includes other chemolithotrophic sulfur oxidizers such as T. ferrooxidans and T. thiooxidans. These characteristics distinguish KU and BC13 from any other species described previously and they thus represent the first acidophilic, thermophilic Thiobacillus species, named T. caldus sp. nov., to be described. The type strain, referred to as strain KU in this paper, has been deposited in the Deutsche Sammlung von Mikroorganismen, Braunschweig, FRG, with the accession number DSM 8584.