Cassidula auris-felis Brug, occurs abundantly on flat, mangrove mud on the west coast of Malaya at a level within three feet of the highest spring tides. Ovotestesof only very few snails lacked developing male gametes but oocytes were often very uncommon, becoming abundant at irregular intervals. Abundant oocytes were found in large proportions of the population after periods of more than eight days without tidal wetting. Abundant oocytes were not found after periods when the mud was left exposed for periods of seven days or less. Statistically, proportions of “predominantly female” snails are shown to be correlated with the number of days without tidal wetting in a complete period of continuous exposure prior to collection. In addition, rather more snails bore abundant oocytes in the lunar “weeks” following new and full moon than in “weeks” following half moons. Conditions leading to abundant oocytes also appear to promote copulation in adults and commencement of gametogenesis in young snails.