Acidobacterium capsulatum, an acidophilic, mesophilic and chemoorganotrophic bacterium, constitutively produced the acid α-glucosidase. The enzyme, which was successively purified to homogeneity by CM-Sepharose, Sephacryl S-300 and Mono-S ion-exchange chromatography, was a monomeric protein, whose molecular weight was estimated to be 65, 000 by gel filtration and sodium dodecylsulfate-polyacrylamide gel electrophoresis. The enzyme exhibited optimum.activity at pH 4.5 and 30°C, being stable in the pH 3.5 to 7.0 region and in the range of 10 to 50°C. No activity was detected above pH 7.5 or above 60°C. Its isoelectric point was 7.0. The enzyme hydrolyzed pnitrophenyla-glycoside, oligosaccharides containing α-1, 3 (nigerose), α-1, 4 (maltose), α-1, 6 (isomaltose), and α-1, β-2 linkages (sucrose), and soluble starch and produced α-configurational glucose. These findings indicate that the A. capsulatum enzyme represents a novel type of α-glucosidase exhibiting a broad substrate specificity. Amino terminal analysis by a protein sequencer provided the sequence of the first eighteen residues as Ser-Ala-Thr-Gly-Ala-Pro-Trp-Trp-Lys-Asn-Ala-Val-Ile-Tyr-Glu-Val-Tyr-Pro.