In this paper,Quantitative real-time reverse transcription-polymerase chain reaction(qRT-PCR) was used to study the tissue-specific changes of NKCC1a mRNA expression by salinity in case of the hybrid(named "GILI" tilapia)by Oreochromis niloticus(♀)×Sarotherodon melanotheron(♂).As a new variety,"GILI" tilapia characterized by high salt tolerance and fast growth is suitable for marine culture.The present results showed that:(1) The NKCC1a mRNA expression level has significant tissue specificity.The NKCC1a gene expressed in all tested tissues of gill,liver,kidney and intestine under salinity below 25.When the salinity increased from 0 to 48,the NKCC1a mRNA expression level showed a positive correlation with salinity in gill(R0.9,P0.01),but nega-tive correlation in intestine and kidney(R≈-0.7,P0.05) and no significant correlation in liver(P0.05).(1)When the salinity raised to 64,the mRNA expression level reached the highest peak after 3 h in gill and intestine,and decreased after 5 h,and there were significant differences between 1 h and 3 h,and between 3 h and 5 h(P0.05).In liver,the maximum level appeared in 5 h and significantly higher than those in 1 h and 3 h(P0.05).In kidney,the expression levels descended continually with the increase of exposure time,but no significant differences were detected in different time(P0.05).Above results revealed that,in the "GILI" fish which acclimates to environ-mental salinity over 25,it is primarily the gill chloride cells to secrete the cation ions and to maintain water and salt balance.So,it could be considered that gill plays a leading role in osmoregulation of "GILI" tilapia.Mean-while,it is expected to revealing the expression mode of NKCC1a gene in euryhaline teleosts in fresh,saline and hypersaline water.