Tilapia is the main cultured fish species in Egypt, contributing 43.5% of farmed fish production and 24% of total fisheries production. The main problem facing tilapia producers is early reproduction before fish reach marketable size, leading to overpopulation and smaller fish at harvest. Approaches to this problem include the use of all-male hybrids and hand-sexing to remove females. An alternative for controlling the effects of unwanted population, polyculture of tilapia with a predator that eats tilapia fry and fingerlings has been proposed by Guerrero (1980), De Graaf (1996), El-Gamal et al. (1998), and Fagbenro (2004). Among the most popular predators used for biological control of tilapia reproduction is the African catfish, Clarias gariepinus. In addition to controlling tilapia reproduction, polyculture increases productivity by a more efficient utilization of the ecological resources in the pond. Stocking two or more complementary species can increase the maximum standing crop of a pond by taking advantage of a wider range of available foods and ecological niches. Nile tilapia, Oreochromis niloticus, is an omnivorous filter feeder and African catfish is considered as a predator targeting fish fry and fingerlings. The aim of this study was to identify the optimal stocking ratio of tilapia and African catfish with respect to water quality, growth and economic performance under Egyptian conditions.