The heteronemertean Baseodiscus mexicanus (Bürger, 1893) was previously known exclusively from the eastern Pacific, except for fragmentary records from Palau and Indonesia, which suggested a possible amphi-Pacific tropical distribution. A single specimen of B. mexicanus collected in Okinawa, Japan, represents the first record of this species from Japanese waters and provides additional evidence for the occurrence of the species in the western Pacific. Baseodiscus mexicanus is strikingly similar in appearance to a sympatric moray eel, Gymnomuraena zebra (Shaw, 1797) (Actinopterygii: Anguilliformes: Muraenidae), possibly indicating a relationship of mimicry between them. [ABSTRACT FROM AUTHOR]