The total content of riboflavin and the ratio of its 3 types were investigated on several kinds of shell-fish. The investigation was made with the whole body, internal organs, shell adductor and other muscles. The riboflavin content was determined by the lumiflavin fluorescence method and 3 types (FAD, FMN and free riboflavin) rotio by paper chromatography. (1) The riboflavin content of internal organs was higher than that in shell adductor and muscles. The vitamin content of the internal organs was higher in shell-fishes with a single shell, such as ear-shell, top-shell and river-snail, than in those with double shells. (2) Among the 3 types of riboflavin, FAD occupied the most part and FMN and free form followed in order of amount. In the shell adductor no free riboflavin was seen at all. But in the internal organs of ear-shell, top-shell and river-snail free riboflavin was the highest, followed by FMN and FAD.