Watercress (Nasturtium officinale), a common spring-brook macrophyte, possesses the glucosinolate-myrosinase system which is regarded as a classic example of chemical defense for terrestrial crucifers. Damage of watercress leaves results in the myrosinase mediated hydrolysis of phenylethyl glucosinolate to the noxious volatile phenylethyl isothiocyanate. Studies at Squabble Brook, CT and Carp Creek, MI showed that cell damaged (frozen) watercress leached substances that were quite toxic to the co-occurring amphipod Gammarus pseudolimnaeus (48-h LC50   1 g(wet/L). These results corresponded well with the toxicity of phenylethyl isothiocyanate to G. pseudolimnaeus (48-h LC50   2 mg/L) and the amount of phenylethyl isothiocyanate yielded from frozen watercress (ca. 3.5 mg phenylethyl isothiocyanate/g watercress). Gammarus strongly preferred senescent yellowed watercress leaves to fresh green leaves in 24 h consumption choice tests, and usually consumed five times more yellowed watercress (>50% of yellowed leaf area vs. 8% of green leaf area), despite the higher nitrogen content of green watercress (2.7% for yellowed vs 5.4% for green). Fresh green watercress contained seven times more phenylethyl glucosinolate than yellowed watercress (8.9 mg/g wet vs 1.2 mg/g) and watercress consumption (area consumed per 24 h) was negatively correlated with glucosinolate concentration (r=0.65; p < 0.05). These results suggest that live watercress is defended against consumption by the glucosinolate-myrosinase system recognized as the principle deterrent system of terrestrial crucifers. This system may be just one of perhaps many examples of the use of defensive chemicals by stream and lake macrophytes.