A comparison was made between the fecundity of female African catfish, Clarias gariepinus, transferred from their natural habitat in Northern Israel to nearby fish ponds and an indoor hatchery respectively, and conspecifics reared and kept in an indoor hatchery in The Netherlands. The results indicate that an inherent endogenous rhythm rather than the natural light periodicity determines the cyclical changes in ovarian activity, and that this internal rhythm is determined by environmental factors at an early stage of development. High water temperatures and the presence of males seem to enhance ovarian activity and to shorten its resting period. The significance of gonadotropin secretion by the pituitary for the ovarian cycle is discussed. It is concluded that for propagation of C. gariepinus throughout the year, independent of the seasons, broodfish should be reared and kept in hatchery tanks at a water temperature of 25°C, with proper feeding, in the presence of males, and at any local light periodicity.