1. Both emergent and submersed growth forms of the amphibious mustard watercress (Nasturtium officinale) contain appreciable concentrations of glucosinolate that deter feeding by a variety of aquatic grazers. Glucosinolate and myrosinase are stored separately in plant tissues, but undergo hydrolysis upon tissue damage to form characteristic isothiocyanates and nitriles. 2. Snails of the genus Physella readily consume yellowed, senescing leaves of watercress, yet are reluctant to attack fresh, green leaves. Laboratory behavioural studies confirm that these preferences are related to intrinsic properties of the leaves independent of periphyton concentrations. Snails are attracted to crushed yellow leaves, yet are actively repulsed by isothiocyanates liberated by crushed, green leaves, despite the higher nitrogen content of the green tissue. 3. The aversion response to crushed green leaves confirms reports by Haynes and Taylor (1984) and others that snails avoid crushed leaves of watercress. However, our interpretation of the interaction is distinctly different from theirs. Rather than a damaged plant signalling imminent danger to host periphyton grazers, the liberation of the compound 2-phenylethyl isothiocyanate from damaged tissues acts as a deterrent that protects the plant against opportunistic herbivores, which include certain snail species.