Pinnatoxins B and C, the most toxic components in the pinnatoxin series, were successfully purified from the Okinawan bivalve Pinna muricata. Their gross structures were determined based on NMR and MS/MS spectral analyses. Their stereostructures were mainly determined by transformation reactions. Pinnatoxins B and C, C-34 epimers of each other, have an amphoteric macrocycle composed of 6,7-spiro, 5,6-bicyclo and 6,5,6-trispiro ketal rings, the same as in pinnatoxins A and D.