A wide variety of bioactive imidazole alkaloids derived from aromatic amino acids were reported from marine invertebrates.1 Representative structures included aplysinopsins,2 topsentins,3 polyandrocarpamide D,4 and na-amidines.5 During the course of our search for antitumor compounds from marine organisms, we isolated a novel brominated (aminoimidazolinyl)indole, designated as discodermindole (1), from the sponge Discodermia polydiscus DuBocage 1879 (family Theonellidae, order Lithistida).6 In in vitro assays, 1 yielded IC50 values of 1.8 μg/mL against P388 (murine leukemia), 4.6 μg/mL against A-549 (human lung), and 12 μg/mL against HT-29 (human colon) cell lines. Its isolation and structure elucidation are reported herein.