The extracellular (1 linked to 3)-beta-D-glucanase [(1 linked to 3)-beta-D-glucan glucanohydrolase, EC 3.2.1.6] produced by Rhizopus arrhizus QU 1032 was purified 305-fold in 70% overall yield. This preparation was found to be homogeneous by ultracentrifugation (sedimentation velocity and equilibrium studies), electrophoresis on acrylamide gel with normal, sodium dodecyl sulfate, and urea-acetic acid gels, and upon isoelectric focusing. The amino acid composition of the enzyme has been determined and it possesses a carbohydrate moiety compose of mannose and galactose (in the ratio approximately 5:1) that is linked to the protein through a 2-acetamido-2-deoxyglucose residue. The molecular weight, as determined by equilibrium sedimentation, is 28,800 and this number was confirmed by electrophoresis on gels of sodium dodecyl sulfate. The enzyme does not possess subunit structure. It hydrolyzes its substrates with retention of configuration and possesses transglycosylating ability. The rates of hydrolysis of a wide variety of substrates were determined, and its action pattern on a series of oligosaccharides containing mixed (1 linked to 3)-, (1 linked to 4)-, and (1 linked to 6)-beta-D-glucopyranosyl residues was investigated. The enzyme favors stretches of beta-D-(1 linked to 3) linkages, but it can hydrolyze beta-D-(1 linked to 4) linkages that are flanked on the non-reducing side with stretches of beta-D-(1 linked to 3) links. The enzyme will not act on (1 linked to 6)-beta-D-glucosyl linkages located in stretches of beta-D-(1 linked to 3) and will not act on (1 linked to 3) beta-D-glycosidic linkages involving sugars other than D-glucose.