To date, many proline-rich cyclic peptides have been discovered from various marine organisms such as sponges, ascidians and so on. These compounds have attracted increasing interest because of their biological activities [1], including cytotoxicity [2], antitubercular activity [3], anti-HIV activity [4], repellent (antifouling) activity [5] and inhibitory activity toward NO production [6]. Among marine creatures, cyanobacteria are known to be prolific producers of peptidic natural products, and several proline-rich cyclic peptides, including wewakapeptin A [7], wewakazole B [8], pahayokolide A [9] and trichormamide A [10], have been isolated from marine cyanobacteria. Against this background, we investigated the secondary metabolites of a marine cyanobacterium Symploca sp. and isolated croissamide (1), a cyclic peptide containing 11 α-amino acids, including five prolines and one N-prenylated tryptophan. Here we report the isolation and structure determination of croissamide (1).