The glycoside hydrolase family (GH) 130 is composed of inverting phosphorylases that catalyze reversible phosphorolysis of beta-D-mannosides. Here we report a glycoside hydrolase as a new member of GH130. Dfer_3176 from Dyadobacter fermentans showed no synthetic activity using alpha-D-mannose 1-phosphate but it released alpha-D-mannose from beta-1,2-mannooligosaccharides with an inversion of the anomeric configuration, indicating that Dfer_3176 is a beta-1,2-mannosidase. Mutational analysis indicated that two glutamic acid residues are critical for the hydrolysis of beta-1,2-mannotriose. The two residues are not conserved among GH130 phosphorylases and are predicted to assist the nucleophilic attack of a water molecule in the hydrolysis of the beta-D-mannosidic bond. (C) 2015 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.