A strictly anaerobic arsenate-respiring bacterium isolated from a gold mine in Bendigo, Victoria, Australia, belonging to the genus Bacillus is described. Cells are Gram-positive, motile rods capable of respiring with arsenate and nitrate as terminal electron acceptors using a variety of substrates, including acetate as the electron donor. Reduction of arsenate to arsenite is catalysed by a membrane-bound arsenate reductase that displays activity over a broad pH range. Synthesis of the enzyme is regulated; maximal activity is obtained when the organism is grown with arsenate as the terminal electron acceptor and no activity is detectable when it is grown with nitrate. Mass of the catalytic subunit was determined to be approximately 87 kDa based on ingel activity stains. The closest phylogenetic relative, based on 16S rRNA gene sequence analysis, is Bacillus arseniciselenatis, but DNA-DNA hybridization experiments clearly show that strain JMM-4(T) represents a novel Bacillus species, for which the name Bacillus macyae sp. nov. is proposed. The type strain is JMM-4(T) (=DSM 16346(T)=JCM 12340(T)).