Manauealide C (1) and anhydrodebromoaplysiatoxin (4), toxic constituents of the Hawaiian red alga, Gracilaria coronopifolia which has been concerned with food poisoning cases, were studied. The absolute structure of manauealide C was determined as 1 by chemical conversion and spectroscopic methods. The first complete assignment of 13C chemical shifts for anhydrodebromoaplysiatoxin (4) was established. The biological activity of 4 was also investigated.