Abstract: Trypsin ⅠmRNA in two species of tilapia(Oreochromis aureus and Oreochromis niloticus) were cloned and sequenced by means of 3′/ 5′2 RACE.The results indicated that the size of trypsin ⅠmRNA of tilapia (O.niloticus) and tilapia (O.aureus) are 858bp and 863bp respectively,with an open reading frame of 738bp,which encoding trypsin containing 245 anmino acid residues with characteristic signal peptides and activation peptides.The tilapia trypsin Ⅰ have highly conserved amino acid residues essential for the catalytic activity and conformational maintenance,which are His,Asp,Ser of catalytic triad and twelve cysteines forming six disulfide bridges,together with the residues determining substrate specificity and generating the S1 substrate-binding pocket of a typical trypsin nature.The identity of trypsin ⅠmRNAof tilapia (O.aureus) compared with that of Antarctic fish (Patanotothenia magellanica),Antlantic Salmon ( Salmo salar),Flounder(Paralichthys olivaceus),Atlantic Cod(Gadusmorhua),Bovine,and Human was 58.3%—72.5 % and 63.3%—76.1%, respectively,And that in O.niloticus was59.1%—73.1% and 63.6%—75.6%.Trypsin I and its mRNA of O.niloticus showed identity of 96.2 % and 99.2% respectively to those of Oreochromis aureus.