Field and laboratory experiments were performed to investigate the palatability to predatory fishes of organic extracts of the Caribbean ascidian Didemnum conchyliatum. This tan-colored compound ascidian grows as an epibiont on seagrass blades. A dichloromethane/methanol extract of the ascidian incorporated into carrageenan food strips at the same volumetric concentration as the extract occurred in the ascidian tissues deterred feeding of a natural assemblage of consumers in the same seagrass beds from which the ascidians were collected. Bioassay-directed fractionation of this extract revealed that the deterrent property was restricted to fractions containing novel indole-maleimide-imidazole alkaloids, didemnimides A to D. Laboratory assays of purified metabolites revealed that didemnimides C and D deterred feeding by a generalist predatory reef fish, but that didemnimides A and B were not deterrent. Only didemnimide D deterred feeding in the field; neither didemnimide C nor the fraction containing didemnimides A and B were deterrent when assayed in the natural environment of the ascidian. Antipredatory activity of the didemnimides was highly dependent on compound structure: only didemnimide D, which bears both a bromine on the indole ring and a methyl group on the imidazole ring, inhibited feeding in both laboratory and field assays. Didemnimide D deterred feeding at approximately 0.035 mg ml(-1), and is among the most potent antipredatory chemical defenses described from tunicates to date.