Three species of snails (Goniobasis livescens, Physa integra, and Lymnaea emarginata) were exposed to acutely sub-lethal concentrations (i.e. 0.2 x 48 hour LC50) of chromium, chlorine, cyanide, copper, lead, phenol, zinc, and acetic acid. They were then exposed to heat shock (i.e. temp. raised from 24 deg. to 36.5 deg. C for G. livescens and P. integra and 24 deg. to 38 deg. C for L. emarginata) both (1) in water with no sub-lethal concentration of the aforementioned chemicals and (2) water with sub-lethal concentration. Controls were also set up in the same fashion. All three snail species tested had significantly higher heat shock mortality when they had been exposed to certain chemicals. However, the degree of response may vary considerably both among species and among toxicants.