A novel thermo-acidophilic bacterium was isolated from an acidic beverage that had the odour of guaiacol. The cells are aerobic, gram-positive, spore-forming rods. The organism, strain TA-67T, grows at temperatures from 20 to 55 degrees C (optimum, 50 degrees C) and at pH values from 2.5 to 5.5 (optimum, pH 3.0). It possesses omega-cyclohexane fatty acid as a major cellular fatty acid. The G+C content of the DNA is 54.1 mol%. Phylogenetic analysis of the 16S rRNA gene sequences indicated that strain TA-67T constituted a distinct lineage in the Alicyclobacillus cluster, with Alicyclobacillus acidoterrestris as the closest neighbour (96.6% homology). Phenotypically, it is similar to, but can be distinguished from, omega-cyclohexane fatty acid-possessing alicyclobacilIi (A. acidoterrestris, Alicyclobacillus acidocaldarius, Alicyclobacillus hesperidum and 'Alicyclobacillus mali') by the morphology of spores and sporangia, by the growth response to different temperatures, and by the profiles for acid production from carbon sources. It is the alicyclobacillus that produces guaiacol, a causative substance for an 'off' flavour of orange juice. On the basis of the phenotypic and phylogenetic evidence, it is concluded that strain TA-67T represents a new species of the genus Alicyclobacillus, for which the name Alicyclobacillus acidiphilus is proposed. The type strain is TA-67T (= DSM 14558T = IAM 14935T = NRIC 6496T).