The yamakagashi, Rhabdophis tigrinus, a common natricine snake of Japan and adjacent East Asia, is generally considered inocuous to man despite its enlarged posterior maxillary teeth and well developed Duvernoy's glands. Three cases of serious envenomation by this species are reported, all marked by delayed, spontaneous, superficial hemorrhaging and profound impairment of normal blood coagulation. In two cases these phenomena were accompanied by signs of severe internal hemorrhaging and hemolysis. Other symptoms may have resulted from transitory involvement of the central and autonomic nervous systems. Therapeutic measures applied in these cases are described, including the apparently effective use of a systemic antihemorrhagic drug. It is concluded that R. tigrinus is a dangerously venomous snake and potentially lethal to man.