Three cultured populat ions (Xianghu , American and Shashi)of Oreochromis nil loticus and one cultured population of O .aureus were examined using RAPD method .12 primers w ere screened out of 20 random primers , and each of the 12 primers revealed inter-or/and int rapopulation genetic variation .There were four species-specif ic amplification bands (sizing 900 , 1500 , 1700 and 730 bp respectively)of primers OPZ06 , OPZ10 , OPZ12 and OPZ19 , each of w hich could be used as molecular genetic markers to identify O .ni lot icus and O .aureus .Genet ic similarity indices (S)suggested that the three populat ions of O .ni loticus maintained the higher deg rees of the int rapopulation genetic variation (S for the three populations of Xianghu , American and Shashi w ere 0 .798 , 0 .795 and 0 .824 respectively), and O .aureus had the smallest degree of genetic variat ion(S =0 .950)because of two times of genetic bot tleneck ef fect .The data suggest that there is the genetic potential of developing new O .niloticus varieties , w hile there is relatively small genetic po tential in O .aureus .Inter-specific genetic distances between O .aureus and three populations of O .ni lot icus w ere 0 .285 , 0 .262 and 0 .344 , respectively , which suggest that the of fspring of O .aureus and Shashi population of O .niloticus w ill develop st ronger heterosis than that of other tw o cross combinat ions .