A novel mesophilic and acidophilic Gram-positive bacterium, designated strain TC-34(T), was isolated from solfataric soil. Strain TC-34(T) grew aerobically at 17-40 degrees C and pH 2.0-6.0, and optimally at 28 degrees C and pH 3.0. Analysis based on 16S rRNA gene sequences showed that strain TC-34(T) was phylogenetically related to members of the genus Alicyclobacillus, with the highest similarity (94.8 %) to Alicyclobacillus pomorum. Strain TC-34(T) showed a range of phenotypic characteristics that differentiated it from recognized Alicyclobacillus species, including growth temperature, assimilation of carbon sources and production of acids from a range of compounds. Strain TC-34(T) was able to oxidize ferrous iron and its growth was correlated with the oxidation of Fe(2+) in culture medium. omega-Alicyclic fatty acids were not detected. On the basis of these results, it was concluded that strain TC-34(T) represents a novel species of the genus Alicyclobacillus, for which the name Alicyclobacillus ferrooxydans (type strain TC-34(T)=JCM 15090(T)=CGMCC 1.6357(T)) is proposed.