Betaine homocysteine methyltransferase (BHMT, EC 2.1.1.5) catalyzes the transfer of a methyl group from betaine to homocysteines (Hcy) to form dimethylglycine and Met, respectively. A full-length cDNA of the BHMT in Lateolabrax japonicus was amplified using RT-PCR and SMART RACE methods. The cDNA of the BHMT in L. japonicus is 1 461 bp in size, with 72 bp 5'-UTR, 183 bp 3'-UTR and 1206 bp ORF, encoding a protein of 401 amino acids with a molecular weight of 44.32 kD and pI 7.21. The sequence analysis indicated that the deduced amino acid sequence of BHMT shared high identity (77%-93%) with nine other species; the highest was 93% with Perca flavescens. Semi-quantitative RT-PCR was used to characterize the expression of BHMT in ten tissues including muscle, heart, eye, brain, gill, liver, intestine, kidney, adipose tissue and spleen. The results showed that BHMT is only expressed in the liver, intestines and kidney. BHMT mRNA in these three tissues declined after the fish were transferred from sea water to a higher salinity environment and induced when transferred to a lower salinity environment. BHMT gene in liver, intestine and kidney can also be induced after intraperitoneal injection of betaine. Our results show that betaine can induce the transcription of BHMT in fish, and BHMT play pivotal roles in adaptation to osmotic change.