We purified an acid trehalase (EC 3.2.1.28, a,alpha,alpha ' -trehalose glucohydrolase) from an acidophilic bacterium, Acidobacterium capsulatum. The enzyme was homogeneous based on polyacrylamide gel electrophoresis, and was composed of a single polypeptide chain with a molecular mass of 57 kDa. Maximum trehalase activity was observed at pH 2.5. The acid trehalase exhibited an apparent K-m of 1.0 mM for trehalose at 30 degreesC and pH 3.0, The trehalase was located in the periplasmic space. The activity of the enzyme was activated by 1.0 mM MnCl2 or CoCl2, and inhibited by 1.0 mM PbCl2, HgCl2, NiCl2, p-chloromercuribenzoate, N-ethylmaleimide, monoiodoacetate, or EDTA. Tile enzyme showed high specificity for trehalose. It was found that an equimolar mixture of alpha -D-glucose and beta -D-glucose was formed on hydrolysis of trehalose by the trehalase.