Tilapia fry viability is one of the important traits because it affects number of seed produced and marketed and, therefore, yield and economic return for hatcheries and the aquaculture industry. In the present study, we examined the effect of female body weight prior to spawning and fry birth weight on survival rates at 20 days of nursing (D20) and at 62 days (D62) (when tagged). Genetic parameters for D20 and D62 were estimated from 86,609 individual fry which were offspring of 63 sires and 77 dams in the sixth generation of a Nile tilapia (Oreochromis niloticus) population undergoing selection for high growth in brackish water environment. Both linear and threshold models were used to estimate genetic parameters for these two traits. The estimates of heritability for D20 after hatching were low and not significantly different from zero across the models studied. By contrast, D62 showed a large heritable additive genetic component on both the liability and observed scales (h2 = 0.24 – 0.79). The maternal and common environmental effects were 6 – 12% for survival at 20d (D20), whereas they were small for survival from 21d to tagging, D62 (1 – 5%). Genetic correlation between D20 and D62 was high and positive (0.72 ± 0.06). However, the estimate was significantly different from one which suggests that D20 and D62 are under different genetic control and should be treated as different traits in selective breeding programs for tilapia. The results from the present study indicate for the first time that there is potential for simultaneous improvement of both survival rates at 20 days of nursing and at tagging in the future genetic improvement program for this tilapia population.