Hydrogen sulphide is poisonous to aerobic organisms and consequently molluscs are rarely found in streams with high concentrations of H2S. In the Arava Desert (Israel) we found a freshwater snail, Melanoides tuberculata, in a stream with a sulphide gradient. The H2S gradient ranged from 7.3 to 0.1 mg/litre, and adult (but not juvenile) M. tuberculata were found to survive in oxygen-depleted water containing hydrogen sulphide concentrations of 34 mg/l. In these micro-habitats M. tuberculata may perhaps breath aerobic oxygen. Juveniles were found further down the gradient and their frequency in the population gradually increased, from 0% at 34 mg/l to 10% at 18 mg/l, and to 56—61% at < 0.1 mg/l. These data suggest either that snails invading the high-sulphide micro-sites do not reproduce, or that they reproduce but the juveniles are unable to survive in the sulphide environment.