1. The occurrence of Aplexa hypnorum (L.) in various habitats on soil types nos. 8 and 37 in Zuid-Beveland depends not only on low salinity but also on other characters of the water. 2. Aplexa hypnorum is adapted to the dry periods on these soil types by the ability of eggs and juveniles of the new generation to survive such conditions. 3. There is only one generation a year. 4. Owing to a dry period in summer, when hatching is considered to cease temporarily, the new generation may be composed of two different size/age classes during autumn, winter, and early spring.