Abstract: Objective To investigate whether there is a competitive relationship between different serotypes of Yersinia enteroclitica(Ye.enterocolitica strains O:3,O:9) by detecting the variation of colony-forming units(CFU) after mixture culture in vitro.Methods The Ye.enterocolitica strains of serotype O:3 and O:9 were mixed at the same quantity in 100 ml Luria-Bertani,meanwhile the two strains were cultured alone as two control groups.The oscillatory liquid were cultured at 28 ℃ and the bacterial suspension of each group was cultured in the plate for colony counting every three hours.Then the monoclonal antibody was used to identify the serotype of each colony.Results The growth peak periods of O:3 serotype in mixtured culture were significantly decreased than those cultured alone.At the culture time of 21,33,and 39 hours,the bacterial counts of the O:3 serotype strains were 4.40,5.81,and 4.79 folds higher than those cultured alone,respectively,and the growth peak perids of O:3 serotype were significantly inbibited by the O:9 serotype.However,no significant difference was observed between the O:9 serotype strain cultured alone and mixed.The multiplication rate of the two serotypes did not show any difference in the mixed and cultured alone cultivation.Conclusion The O:3 serotype did not inhibit the reproduction rate of O:9.But the O:9 serotype could inhibit the O:3 serotype.