Shellfish of the genus Pinna live mainly in shallow areas of the temperate and tropical zones of the Indian and Pacific Oceans. The adductor muscle of this bivalve is eaten in Japan and China, and food poisoning resulting from its ingestion occurs frequently. Chinese investigators have reported that the toxic extract from Pinna attenuata, referred to as pinnatoxin, is a Ca²⁺ channel activator. We report here the isolation of two toxins from Pinna muricata, viz. pinnatoxin A (1) and B, and describe the gross structure of 1, a novel polyether macrocycle with carboxylate and iminium functionalities.