We cloned complementary DNA (cDNA) encoding aquaporin 1 (AQP1) and arginine vasotocin receptor (AVT-R) from gill and kidney tissue of the black porgy (Acanthopagrus schlegeli), respectively. Black porgy AQP1 cDNA consists of 786 base pairs (bp) and encodes a protein of 261 amino acids, and AVT-R partial cDNA consists of 606 bp. To investigate the osmoregulatory abilities of black porgy in different salinities (35‰ seawater, SW, 10‰ SW, freshwater, FW), we examined the expression of AQP1 and AVT-R mRNA in osmoregulatory organs using the reverse transcription polymerase chain reaction (RT-PCR). AQP1 mRNA levels increased in the gill and intestine during FW acclimation, and the mRNA expression in the kidney was greatest in 10‰ SW and then decreased in FW. On the other hand, AVT-R mRNA was expressed in the gill only in 10‰ SW, while it increased in the kidney in 10‰ SW and then decreased in FW. Thus, the expression of these mRNAs increased in hypoosmotic environments. These results suggest that AQP1 and AVT-R genes play important roles in hormonal regulation in osmoregulatory organs, thereby improving the hyperosmoregulatory ability of black porgy in hypoosmotic environments.