The genetic polymorphisms of the esterase-4 (Es-4) and esterase-6 (Es-6) were studied in Korean (Seoul) rat (Rattus norvegicus) population using starch gel and polyarcrylamide gel isoelectric focusing electrophoresis. the four phenotypes of Es-4, Es-4AB(14.89%), Es-4B(76.60%), Es-4BC(6.38%), and Es-4C(2.13%) were found. The allele frequencies of Es-4*a, Es-4*b and Es-4*c were 0.075, 0.872 and 0.053, respectively. the seven phenotypes of Es-6, Es-6A(39.58%), Es-6AB(22.94%), Es-6B(12.50%), Es-6BC(6.25%), Es-6AC(12.50%), Es-6AD(2.08%), and Es-6BD(4.17%) were found. The allele frequencies of Es-6*a, Es-6*b, Es-6*c and es-6*d were 0.583, 0.292, 0.094 and 0.031, respectively. In this study, it was also observed that Es-4 and Es-6 showed tissue specific polymorphisms.