Optical and proton NMR spectral studies of the aplysiatoxins and derivatives, degradation products of the toxins, and an X-ray crystallographic analysis of 19,21-dibromoaplysiatoxin show that the absolute configurations of the ten asymmetric carbons are 3S,4R,7S,9S,10S,11R,12S,15S,29R,30R. The 31-nor compound, oscillatoxin A, has the same absolute stereochemistry at C(3), C(4), C(7), C(9), C(10), C(11), C(12), C(15), and C(29). Evidence is presented which shows that the aplysiatoxins, oscillatoxin A, and the corresponding anhydrotoxins have relative and absolute stereochemistries depicted in the structural formulae.