1. A critical survey of the species of Opisthobranchia previously recorded from Japanese waters results in the enumeration of 73 valid forms. To this number about 66 have been added by me. 2. A detailed analysis of the Japanese Opisthobranch fauna shows that this latter has several littoral species in common with the Atlantic and Mediterranean faunas; that it is closely related to the Red Sea fauna, more closely to the Indo-Pacific fauna; further, that it is not entirely distinct from the fauna of acific North America. 3. A comprehensive list of the species of Pleurocoela, Sacoglossa and Acoela known to occur in Japanese waters is given. The following three are placed on record as new species: Runcina elioti, Tethys kurodai and Stiliger berghi. 4. The following species has been recorded previously from the Atlantic and the Mediterranean and so has its range extended eastward: Hermaea dendritira. 5. Complete coloured sketches from living specimens have been given for the first time for the previously noted forms: Aglaia gigliolii, A. cyanea, Petalifera punctulata and Elysia japonica. Additional coloured sketches have been given of T. parvula, Hermaea dendritica and Berthella plumula delicata.