Two species of pulmonate snails, Lymnaea catascopium and Physa integra, were collected from Douglas and Houghton Lakes. Snail populations from the former lake (pristine) had never been exposed to copper sulfate, whereas those from the latter one (treated) had been subjected to the molluscicide for more than 40 years. Molluscs from the treated lake were more resistant to the copper at concentrations of 0.5 and 1.0 ppm than those from the pristine lake. Furthermore, larger lymnaeids had a higher survival rate than smaller ones.