The ability of acid tolerance is one of the most important properties of lactic acid bacteria.To investigate the gene expression of three stains of Lactobacillus casei which incubated in different acidic condition,a semi-quantitative RT-PCR method was used to detect the H⁺-ATPase gene expression in different acidic condition.The result indicated that the growth of Lactobacillus casei had been restrained by the acid stress，especially when the incubated condition changed to pH4.0,and the expression of H⁺-ATPase gene was increased along with the increased of incubated condition acidity.We guess it has some relationship between H⁺-ATPase and the growth ability of Lactobacillus casei in different acidic condition.