en Biology & physiology Biology & physiology پژوهشي Orginal research papers The experiment was conducted to determine the effect of salinity and dietary protein levels on &nbsp;physiological and haematological changes as well as growth of the juveniles of hybrid tilapia (<em>Oreochromis mossambicus&times;Oreochromis niloticus</em>). Fish were stocked at 20 fish 50L^-1. The initial average weight of the juveniles was 2.1&plusmn;0.02 g and they were kept in salinities 0, 10, 20 and 35 ppt (S1, S2, S3 and S4, respectively) and fed with 30% (P1) and 40% (P2) dietary protein levels for 45 days. The harvesting weight and SGR were not significantly (<em>p</em>>0.05) different between S2 and S3 treatments for 30% to 40% protein levels varied in connecting S2-S3 with P1diets or in between S2-S3 with P2 diets. The food consumption significantly (<em>p</em><0.05) increased with decreasing protein levels (40% to 30%) in diets at 10-20 ppt salinities. The survival rates were not significantly (<em>p</em>>0.05) different between 10-20 ppt salinities with P1 and P2 diets. The lowest FCR and highest muscular hydration were achieved at intermediate salinities with low protein diets. No obvious changes occurred in Hct%, Hb, plasma protein, muscles and liver protein levels in all treatments. Furthermore, the plasma cortisol and glucose levels tended to increase at higher salinities but were not significantly (<em>p</em>>0.05) different among all treatments. The physico-chemical parameters of water (i.e. dissolved oxygen: DO, temperature, ammonia etc.) were at a normal range throughout the study. Results showed that environmental salinity (0-35 ppt) had no adverse effects on growth and biochemical changes and also no required high protein levels in diets at any salinity. Hybrid tilapia, Haematology, Physiology, Cortisol, Muscular hydration 1268 1279 http://jifro.ir/browse.php?a_code=A-10-1663-1&slc_lang=en&sid=1 J. Mian maviamian@gmail.com 100319475328460034051 100319475328460034051 Yes University of Karachi, Pakistan P.Z.J.A. Siddiqui 100319475328460034052 100319475328460034052 No University of Karachi, Pakistan