An emended description of Amphidiniopsis arenaria Hoppenrath 2000, based on material from the Sea of Japan.

Cells of a taxon similar to the type species of Amphidiniopsis, Amphidiniopsis kofoidii, were found during the course of a study of marine sand-dwelling dinoflagellates in Peter the Great Bay, Sea of Japan, Russia. These specimens were examined in detail using light and scanning electron microscopy and were identified as Amphidiniopsis arenaria, a species known so far only from the North Sea, Germany. Morphological variability within the species and details of its sulcal construction are described here for the first time.