Toxicity tests on the whole nematode community from Restronguet Creek, a severely contaminated estuary, show that nematodes are more resistant to copper than those from an adjacent, less contaminated estuary. This is a result of an increase in the abundance of Curesistant species, the evolution of enhanced Cu tolerance in some species and the probable exclusion of more sensitive species. These findings are in agreement with extensive ecological studies (Somerfield et al. (1994) Mar. Ecol. Prog. Ser. 105, 79-88; Millward, pers. obs.) which suggest that the Restronguet Creek nematode community is distinct from that in the Percuil River, and supports the inference that this distinction is a product of the Cu contamination. The study demonstrates for the first time that pollution-induced community tolerance (PICT) may be used as a tool to evaluate the biological impact of a chronic pollutant on marine benthos, and that ecologically meaningful results can be gained from a simple and cheap acute toxicity test. The natural diversity of the nematode fauna makes this group particularly suitable for the application of PICT.