During the biological survey by the R/V Sohgen-Maru, a research vessel of the Marine Biotechnology Institute (M.B.I.), in the course of research on opisthobranchs, two individuals of a previously unknown ascoglossan opisthobranch were collected in the intertidal zone of Carp Island of the Palau Islands on August 19, 1990. These specimens were found among the thalli of green algae, Halimeda macroloba or Udotea sp. ; unfortunately, however, it was impossible to ascertain which alga was the proper host to this species. Detailed and careful examinations on these specimens revealed that they represented clearly a new species belonging to an unknown genus.