We have been studying Anisakidae larvae, their intermediate hosts and their final hosts in the northern Japan Sea area. These larvae cause anisakidosis. According to the investigation, the recent burst of pseudoterranovosis in this area can be attributed to the increased presence of sea lions, which proliferate in the Arctic region, then migrate to the northern Japan Sea and eat the intermediate host fish. In a stomach of a male sea lion that was captured in February 1995, we found more than 4,500 Pseudoterranova decipiens. Although there is no known circumstance in which a human would consume an adult worm of Anisakis nematode, an astonishing case of this was found in Kanazawa; a female young adult Pseudoterranova decipiens undergoing the final metamorphosis was emitted from a patient. This indicates that the Anisakis larva can mature into the adult worm in humans. It is postulated that the Pseudoterranova decipiens larva is in the process of adapting to use humans as the final host.