While investigating the role of amphipod shredders on litter (watercress, Nasturtium officinale) decomposition, I learned that Nasturtium, after it had been frozen, was toxic to Gammarus pseudolimnaeus. Observations were made and experiments were then performed on Gammarus and watercress from two springbrooks: Carp Creek, MI (in Aug) and Squabble Brook, CT (Oct-Nov). Toxicity tests with fresh frozen watercress leaves gave 48 h LC50 values (trimmed Spearman-Karber methods) or 475 mg wet tissue/L (95% CI=402-528 mg/L) in MI and 1122 mg/L (996-1262 mg/L) in CT. These differences may be due to either temporal or geographic variability. Watercress, a crucifer, is known to contain glucosinolates, which upon tissue damage are hydrolyzed to isothiocyanates or nitriles. A 2 degree compound, phenylethylisothiocyanate, which occurs in watercress was found to be quite toxic in solution. Forty-eight h LC50 values for phenylethylisothiocyanate were 3.62 ug/L (95% CI=2.77-4.74 ug/L) in MI and 0.96 ug/L (0.67-1.36 ug/L) in CT. Choice (preference) trials in CT indicated little consumption of fresh green watercress leaf disks. Overall, less than 10% of the fresh cress offered was consumed (% area remaining = 94.7% for 12 h starvation; 92.5% for 48 h starvation). Yellow (light deprived) cress was consumed much more readily, with only 51.4% remaining for 12 h starvation and 30.8% remaining for 48 h starvation. The most fresh cress was consumed when it was the only choice (about 90% remaining) and the least was consumed when there was an equal or greater number of yellow disks (2:2 or 2:1, ca 94%). These results suggest that watercress may possess defensive chemicals which reduce herbivory by invertebrates such as Gammarus.