The common intertidal hermit crab Pagurus granosimanus learns in one or two trials to reject an attractive, novel food (beef) when illness is induced by lithium chloride injected one hour after the animal accepts and eats the beef. Crabs fed a familiar food (fish) before lithium chloride injection do not learn to avoid the fish. Nor do they learn to reject beef when injected with a sodium chloride solution, or when punctured with a hypodermic needle one hour after their first and second beef meals. Because many crustaceans are scavengers and generalist feeders, they must commonly encounter a wide variety of toxic foods. Quickly acquired and long-lasting aversion to a new food eaten a few hours before the onset of a serious physiological upset could cause these animals to avoid such hazardous foods in the future. Food aversion learning has never before been reported in a crustacean.