Gonad development, reproductive cycle and sex ratio of Thais carinifera were studied through a histological investigation of its gonads. Samples of Thais carinifera were monthly collected during the period from February 2007
to May 2008, from Lake Timsah, Suez Canal, Egypt. Staging criteria for the gonad of male and female snails were described. The ovary and testis in both sexes were composed of a number of oogenic follicles and several
spermatogenic follicles respectively. Reproductive cycle of Thais carinifera could be categorized into five stages: early active, late active, ripe, fully ripe and partially spawning phase (in males), and, early active, late active, ripe, spawned
and spent phase (in females). Sex ratio departed from 1:1 and the males dominated the population.