A β-mannosidase was purified to homogeneity from the culture filtrate of Aspergillus niger. A specific activity of 500 nkat mg^-1 and a 53-fold purification was achieved using ammonium sulfate precipitation, anion- exchange chromatography, and gel filtration. The isolated enzyme has an isoelectric point of 5.0 and appears to be a dimer composed of two 135-kDa subunits. It is a glycoprotein and contains 17% N-linked carbohydrate by weight. Maximal activity was observed at pH 2.4-5.0 and at 70°C. The β- mannosidase hydrolyzed β-1,4-linked manno-oligosaccharides of degree of polymerization (DP) 2-6 and also released mannose from polymeric ivory nut mannan and galactomannan. The K(m) and V(max) values for p-nitrophenyl-β-D- mannopyranoside were 0.30 mM and 500 nkat mg^-1, respectively. Hydrolysis of D-galactose substituted manno-oligosaccharides showed that the β-mannosidase was able to cleave up to, but not beyond, a side group. An internal peptide sequence of 15 amino acids was highly similar to that of an Aspergillus aculeatus β-mannosidase belonging to family 2 of glycosyl hydrolases.