Three strains of Egyptian Nile tilapia Oreochromis niloticus (Fayom (F), Kafr El-Sheikh (K) and Abbassa (AB) were used to study the effect of salinity stress on the growth performance. Yolk sac fries were collected from each strain to reduce the effect of age. The fish with mean initial weight 1.94 ± 0.06 (AB), 1.95 ± 0.06 (F) 1.93 ± 0.08 (K) were transferred directly from fresh water to three levels from salinity 0, 5 and 15 ppt in aquaria (three replicates for each strain and salinity) for 96 hrs. Ten fish per each aquarium were collected after the stress from each genotype to grow in saline water or freshwater. All tested fish in saline and freshwater fed twice daily with commercial feed pellets containing 32% protein. All strains demonstrated significant variation (P