Various manno-oligosaccharides including alpha-D-man-(1-->2)-D-man and alpha-D-man-(1-->2)-alpha-D-man-(1-->2)-D-man were formed when a highly concentrated mannose solution was incubated in the presence of alpha-mannosidase from Aspergillus niger. alpha-D-Man-(1-->2)-D-man and alpha-D-man-(1-->2)-alpha-D-man-(1-->2)-D-man were isolated by activated carbon chromatography followed by high performance liquid chromatography using an amino-silica column. In addition to the above oligosaccharides, alpha-D-man-(1-->3)-D-man, alpha-D-man-(1 -->6)-D-man, and alpha-D-man-(1-->2)-alpha-D-man-(1-->6)-D-man were also isolated.