We investigated the potential role of defensive chemicals in the avoidance of watercress (Nasturtium officinale) by the amphipod Gammarus pseudolimnaeus with observations and experiments at two springbrooks: Carp Creek, MI (in Aug) and Squabble Brook, CT (Oct-Nov). Fresh frozen watercress was toxic to the amphipods. Toxicity tests gave 48 h LC50 values of 475 mg wet tissue/L (95% CI=402-528 mg/L) in MI and 1122 mg/L (996-1262 mg/L) in CT. A secondary compound, phenylethyl isothiocyanate, which occurs in watercress was quite toxic in solution. Forty-eight h LC50 values were 3.6 ug/L (2.8-4.7 ug/L) in MI and 1 ug/L (0.7-1.4 ug/L) in CT. Choice (preference) trials in CT indicated little consumption of fresh green watercress leaf disks in 24 h; overall, <10% of the fresh cress offered was consumed. About 60% of yellowed (light deprived) was consumed. Frozen cress that was preleached, was readily consumed, indicating loss of the toxic compounds. These results and associated chemical analyses suggest that watercress may posses defensive chemicals which reduce herbivory by aquatic invertebrates such as Gammarus. This may be just one of perhaps many examples of the use of defensive chemicals by stream and lake macrophytes.