We investigated the potential role of defensive chemicals in the avoidance of watercress (Nasturtium officinale) by the cooccurring amphipod, Gammarus pseudolimnaeus at two spring brooks: Carp Creek, Michigan and Squabble Brook, Connecticut. We conducted observations and laboratory experiments on the consumption of watercress, the toxicity of damaged (frozen) watercress, and the toxicity of damage-released secondary chemicals. Field-collected yellowed watercress typically lacked the bite and odor characteristic of green watercress and was consumed by G. pseudolimnaeus. G. psuedolimnaeus strongly preferred yellowed watercress to green watercress despite the higher nitrogen content of the latter (2.7 vs 5.4%), and usually consumed five times more yellowed watercress (> 50% of yellowed leaf area vs. < 8% of green leaf area presented). Fresh green watercress contained seven times more phenylethyl glucosinolate than yellowed watercress (8.9 mg/g wet vs. 1.2 mg/g). Cell-damaged (frozen) watercress was toxic to G. pseudolimnaeus (48-hr LC50s: ca. 1 g wet/liter), and the primary volatile secondary chemicals released by damage were highly toxic. The predominant glucosinolate hydrolysis product, 2-phenylethyl isothiocyanate had 48-hr LC50s between 0.96 and 3.62 mg/liter, whereas 3-phenylpropionitrile was less toxic, with 48-hr LC50s between 130 and 211 mg/liter. These results suggest that live watercress is chemically defended against consumption. The glucosinolate-myrosinase system, recognized as the principle deterrent system of terrestrial crucifers, is also possessed by N. officinale and may contribute to defense from herbivory by aquatic crustaceans. This system may be just one of many examples of the use of defensive chemicals by stream and lake macrophytes.