Four new alkaloids, didemnimides A-D (1-4), possessing a novel indole-maleimide-imidazole carbon skeleton, have been identified as the major predator deterrents found in the chemically-defended Caribbean mangrove ascidian Didemnum conchyliatum. The carbon skeleton of the didemnimides was identified by X-ray analysis of didemnimide A (1), while the structures of the related didemnimides B-D were subsequently assigned using combined spectral methods that emphasized one- and two-dimensional NMR methods. The didemnimides, in particular didemnimide D (4), are potent feeding deterrents against a natural assemblage of mangrove-specific carnivorous fish.