Inducible defenses are phenotypically plastic traits in which individuals or colonies develop a predator/herbivore-resistant trait in response to a cue. The barnacle Chthamalus fissus, commonly found in the upper intertidal of southern California and Baja California, Mexico, exhibits three morphs: one with an oval operculum, one with a narrow, slit-like operculum, and a relatively uncommon bent form with the operculum opening on one side. A previous study suggested that the narrow and bent morphs are defended from attack by the predatory snail Mexcanthina lugubris lugubris. In the present study, predator exposure and predator exclusion experiments revealed that operculum morphology of C. fissus is a plastic trait: individuals exposed to M. lugubris lugubris develop into the narrow operculum morph or, less commonly, the bent morph. While some species of marine invertebrates exhibit either a generalized plasticity response to various predators or plastic traits that are specific to the type of predator to which they have been exposed, the observation in this study appears to be the first demonstration of the occurrence of alternative inducible defenses to the same predator within a single species of marine invertebrate.