The absolute stereochemistry of the sponge metabolite cyclosmenospongine (1) was determined as 5R, 8S, 9R, 10S by chemical correlation. Substitution of the methoxyl group by an amino group in the cyclic product 3 of acid-catalyzed rearrangement of ilimaquinone (5) afforded cyclosmenospongine 1. Cyclosmenospongine was also obtained by acid-catalyzed cyclization of smenospongine (6).