The polymorphism of some enzymes and allele frequencies were investigated in Korean wild black rat(Rattus rattus) population. The results are as follows LDHA: Two common alleles, LDHA1 and LDHA2 were found, and the allele frequencies of LDHA1 and LDHA2 were 0.670 and 0.321, respectively. Es-4: Two common alleles were Es-4a and Es-4b. The allele frequencies of Es-4a and Es-4b were 0.893 and 0.107, respectively. Es-7: There were two alleles, Es-7a and Es-7b. The allele frequencies of Es-7a and Es-7b were 0.929 and 0.071, respectively. Es-8: There were two alleles, Es-8a and Es-8b. The allele frequencies of Es-8a and Es-8b were 0.893 and 0.107, respectively. Es-10: There were two alleles, Es-10a and Es-10b. The allele frequencies of Es-10a and Es-10b were 0.107 and 0.893, respectively. Es-2, Es-6, LDHB, Tf, Alb, FH and PGM are shown only one phenotype each of all. Therefore, these proteins seem to be monomorophic in Korean wild black rat population. And, the four esterase (Es-6, Es-7, Es-8 and Es-10) systems were firstly investigated in wild black rat population.