To understand fish osmotic adjustment mechanisms in saline and alkaline water, the partial cDNA sequence was obtained from gills of . Physiological changes in serum osmolality, ion concentration (Na⁺, K⁺, Cl^- and Ca²⁺), and gill carbonic anhydrase (CA) activities were determined, and mRNA gene expressions under saline (10 g/L, 15 g/L NaCl), alkaline (1.5 g/L and 3 g/L NaHCO₃), and saline-alkaline (salinity 10, 15 g/L NaCl; salinity 1.5, 3 g/L NaHCO₃) conditions at different times (0 h, 6 h, 12 h, 24 h, 48 h, 72 h and 96 h) were compared. The results showed that serum osmolality, ion concentration, gill CA activity, mRNA gene expression correlated positively with the strength of saline, alkaline and saline-alkaline stress. Over time, serum osmolality and ion concentration trends increased and then decreased. Osmotic pressure insaline and saline-alkaline water was higher than that in alkaline water. Gill CA activity in alkaline and saline-alkaline water was higher than that in saline water. Under low concentrations of stressors, CA activity reached its highest level at a later time. Slightly higher gene mRNA expression was detected in gills under high concentrations of stressors ( mRNA expression in saline, alkaline and saline-alkaline water was increased, but the increase was more evident in alkaline and saline-alkaline water (<0.05). The results showed that CA and NBCe1 in Nile tilapia are involved in salinity and alkalinity regulation under osmotic stress. The results provide a basic understanding of the physiological regulation during salinity-alkalinity adaptation.