The reaction conditions for synthesis of various alkyl beta-mannosides by transmannosylation with Aspergillus niger beta-mannosidase were studied. Maximum yield of methyl beta-mannoside expressed in mol% based on initial mannobiose was 81% in the presence of 50% (v/v) methanol. The longer carbon chains were, the less the yield of corresponding mannoside became. Octyl beta-mannoside, a new glycoside, was successfully synthesized in a 2 mol% yield in the presence of 90% (v/v) octanol.