We recently have reported isolation of stellettazole A (1), an antibacterial homosesquiterpene amide of 1-guanidinopropyl-3-methylhistamine, from a Stelletta sponge collected from Shikine-jima island, 200 km south of Tokyo.2 Interestingly, it inhibited Ca2+/calmodulin-dependent phosphodiesterase. Therefore, we continued to examine the sponge extract, which resulted in the isolation of two dimeric stellettadines, bistellettadines A (2) and B (3). Stellettadine A (4) is a sesquiterpene amide of 1,4-diguanidinobutane possessing larval metamorphosis-inducing activity in an ascidian, which we had isolated from another Stelletta sponge.3 In this paper, we describe isolation, structure determination, and biological activities of these new metabolites.