© 2015 Federation of European Biochemical Societies.The glycoside hydrolase family (GH) 130 is composed of inverting phosphorylases that catalyze reversible phosphorolysis of β-d-mannosides. Here we report a glycoside hydrolase as a new member of GH130. Dfer-3176 from Dyadobacter fermentans showed no synthetic activity using α-d-mannose 1-phosphate but it released α-d-mannose from β-1,2-mannooligosaccharides with an inversion of the anomeric configuration, indicating that Dfer-3176 is a β-1,2-mannosidase. Mutational analysis indicated that two glutamic acid residues are critical for the hydrolysis of β-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 β-d-mannosidic bond.