Leuconostoc mesenteroides was found to produce a remarkable amount of riboflavinylglucoside when grown on a sucrose medium containing riboflavin, but not on any other kinds of sugar media. The formation of riboflavinylglucoside was observed to depend largely on the kinds of seed culture and on the temperature of fermentation and also on the concentration of sucrose. Riboflavin compounds of oligosaccharides were observed to be formed by successive transfers of D-glucosyl group of sucrose to riboflavin during the fermentation.