A new chemolithotrophic bacterium unable to grow autotrophically is described. Its growth occurs with yeast extract or casein hydrolysate and is greatly enhaced by the addition of thiosulfate or a variety of individual organic compounds. Growth also occurs in a mineral salts medium to which both thiosulfate and an appropriate single organic substrate are added but not with either supplement alone. Elemental sulfur, thiosulfate and tetrathionate are oxidized to sulfate. The taxonomic position of the organism is briefly discussed and the name Thiobacillus perometabolis is proposed.