This study was conducted to investigate the ability of common carp, Cyprinus carpio, to adapt to various salinity levels. Two experiments were conducted to study the effects of gradual increase of salinity on weight gain, survival, immunological and osmoregulatory responses and consequent resistance to Vibrio vulnificus infections in common carp. In the first experiment, sodium chloride was added to the water by 0.5ppt every two days to reach a final concentration of 4, 6 and 8ppt, and fish were sampled after 14 days. In the second experiment, sodium chloride was added by 0.5ppt every week to reach a final concentration of 6 ppt and fish were sampled after one month. Fish of the first experiment exposed to 8ppt salinity showed significant increase in all serum measurements and detrimental effects on weight gain and survival. In the second adaptation experiment, the unaltered glucose, cortisol and ions levels indicated that salinity up to 6ppt did not produce significant stress in common carp it decrease the mortality rate caused by V. vulnificus. The above results suggest that common carp, a freshwater fish, can withstand gradual increases of salinity of up to 6ppt, and thus can be raised in brackish water with little or no detrimental effects on growth and performance.