Oscillatoxin A, a major toxic metabolite of a mixture of Oscillatoria nigroviridis and Schizothrix calcicola from Enewetak, has been identified from high-frequency 'H and 13C NMR studies as 31-nordebromoaplysiatoxin. Three minor bromine-containing toxic compounds from this algal mixture, viz., 21-bromooscillatoxin A, 19,21-dibromooscillatoxin A, and 19-bromoaplysiatoxin, have also been identified. Photolysis of N-(3-methyl-2-butenyl)phthalimide (1) in methanol gave cyclic compounds 2a and 3a probably via successive processes; intramolecular electron transfer (1 - 19), polar addition of methanol (19 - 20), and cyclization of diradical(20 - 2a + 3a).