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Not AvailableThe euryhaline white shrimp Penaeus vannamei lives in both coastal and oceanic areas and is capable of surviving over a large range of salinities. There is an ambiguity on its osmoregulation pattern and variations in its tol- erance to low salinity waters. In order to determine the effects of salinity on growth, survival, osmotic regulation and ionic composition (Na+, K+, Ca++ and Mg++), the juveniles of P. vannamei were divided into five treat- ments (1, 5, 7, 15 and 25 ppt) in triplicates for conducting the experiment. The experiment was carried out over a period of 3 weeks with two sampling time points (8th and 21st day). No significant differences (P > 0.05) were found in feed conversion rate, survival percentage, body weight gain percentage and specific growth rate. The isosmotic point estimated was 671.3 mOsm/kg (21.1 ppt) which is the lowest value obtained for the genus Penaeus so far. The hemolymph osmolality was regulated within a narrow range for wide variations in salinity. Changes in hemolymph Na+ and Mg++ concentrations paralleled those of hemolymph osmolality; values were around significantly higher at 15 ppt than other treatments. Whereas, K+ and Ca++ were present at higher concentrations than Mg++ and lower than Na+ but didn't show any trend with the hemolymph osmolality. It suggested that total calcium levels are maintained regardless of salinity and may not bear a connection with osmoregulatory mechanisms. Among the four ions determined, the sodium was found at highest concentration in hemolymph followed by calcium, potassium and magnesium. Our findings suggest that low salinity does not seem to affect osmotic regulation to the extent so that growth and survival rate will be affected in P. vannamei. If the acclimation process is followed properly, the species has excellent potential in inland saline waters at salinity as low as 1 ppt.Not Availabl