Abstract: The prolactinⅠ(PRLⅠ) genes were cloned from pituitary glands of Oreochromis mossambicus, O.hornorum and their hybrids by homology cloning approach. The cDNA sequences and amino acid sequences deduced from them were analyzed through alignment. The results show that PRLⅠgenes from the four different species of tilapia consisted of 798 bp including open reading frame (ORF) of 639 bp, and encoded a putative protein of 212 amino acids. There was only amino acid residue difference between O.mossambicus and O.hormorum at the 31^st site. The expression of PRLⅠgenes in different tissues and organs of O.mossambicus and O.hornorum as well as the relative expression of PRLⅠgenes of the four species of tilapia under salt stress were analyzed by real time quantitative PCR (qPCR). The results reveal that PRLⅠtranscript was detected at high level in pituitary and low level in other tissues and organs. The expression of PRLⅠdecreased significantly after exposure to salt stress in pituitary. These results suggest that PRLⅠgenes play an important role in the osmoregulation of tilapia.