The competitive survival mechanism of Lactobacillus casei YIT9018 has been studied using Escherichia coli LDTMI in MRS and TGY media. The results obtained are summarized as follows; 1. When two oragisms were cultured together, the growth of E. coli was inhibited greatly by L. casei in MRS medium, but in TGY medium, E. coli was inhibited only slightly. 2. The growth inhibition of E. coli by L. casei was studied at the controlled pHs (pH 7, 6, 5, and 4) of MRS broth. The growth inhibition of E. coli was not distinct at pH 7 and 6, but at pH 5 and 4, E. coli was inhibited distinctively. This result means L. casei produced E. coli-inhibiting substances during its growth below pH 6.0 in MRS broth. 3. The growth inhibition of E. coli by L. casei was analyzed by cylinder plate method. MRS broth adjusted to pH 5 exhibited the inhibition zone of 4.2 ㎜ diameter against E. coli while L. casei cultured MRS broth exhibited the inhibition zone of 10 ㎜ diameter at the same pH. Inhibition zone exhibited by L. casei cultured MRS broth was the largest one among the three cultured media. 1t is apparent that production of E. coli-inhibiting substances (LC factor) by L. casei in addition to the acid production. 4. Sodium acetate in MRS medium was found to play the important role in the synthesis of LC factor by L. casei The role of sodium acetate was also evidenced in TGY medium by exhibiting the increased inhibitory activity when TGY medium was supplemented with sodium acetate and 0.2 % glucose. 5. L. casei YIT9018 was found to compete and multiply in the natural habitat by producing acid as well as LC factor.