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Evidence is presented that sex determination in Oreochromis niloticus (L.) has a multifactorial genetic determination. Families of O. niloticus reared in separate suspended net enclosures and cages in the same pond using the same procedures were sexed and weighed at 14 weeks of age. Heritability of sex ratio was estimated by the method of Bull, Vogt & Bulmer (1982) to be 0-26 (95% confidence interval = 0-13-0-48). The heritability of sexual dimorphism, the amount of deviation between weights of male and female siblings, was estimated by standard variance component analysis to be zero. Correlations between sex ratio and parameters of size of each family are given. The results are discussed in the context of earlier reports on changes in sex ratio of tilapia species in response to temperature changes. The implications of these results for genetic improvement of growth rate of tilapia are discussed.