During the summers of 1962 and 1963 the Geologisch en Mineralogisch Instituut of Leiden University and the Rijksmuseum van Natuurlijke Historie at Leiden carried out a joint exploration of the geology, oceanography and biology of Ria de Arosa, a fiord-like bay in northwestern Spain, north of Vigo. These investigations were not confined to the Ria proper, but also included the affluent rivers like the Rio Ulla and Rio Umia. These explorations will be continued in 1964. In July 1963 a specimen of a peculiar isopod was found by the oceanographic party when exploring the Rio Ulla, a river emptying in the northeastern corner of the Ria de Arosa. This find caused the biologists to give more attention to the locality where the animal was caught and many more specimens were obtained there during a trip specially organized for the purpose. A study of the collected material revealed that the isopods are Valvifera belonging to the subfamily Mesidoteinae of the family Idoteidae. However, they could not be fitted into any of the known genera of this subfamily and consequently are described here as constituting a new genus and species. The facts that (1) the species is not rare in the type locality, (2) the specimens are of good size, measuring up to 15 mm, and (3) they inhabit a habitat which is neither unusual nor difficult to explore, make it difficult to understand why the present form has not been discovered before. As the type locality is not very close to any big harbour, it seems unlikely that the species has been accidentally introduced there from somewhere else. A thorough exploration of other brackish waters along the European southwest coast may show that the species also occurs elsewhere in that area.