Many aquatic snails occupy ephemeral habitats that are occasionally subjected to severe environmental conditions. To investigate the physiological capacity of these animals to resist extreme environmental changes, we aerially exposed aquatic pond snails (Physella integra) to temperatures of 5°C and 20°C for 6, 8, 12, 24, 36, 48, and 60 hours. Survivorship varied with temperature and exposure times. At 20°C, survivorship was 0% for snails aerially exposed for 24 hours or longer. At 5°C, 0% survivorship was attained at 60 hours exposure. Sensitivity to aerial exposure was related to shell size of individuals, with larger specimens (shell length greater than 7 mm) significantly more likely to survive exposure than smaller specimens at temperatures and intervals with incomplete mortality. These results suggest that epiphragm development and size-specific survivorship predispose these animals to rapid population recovery following severe, short-term environmental fluctuations.