In order to study effect of the phage ${phi}$ FSV on the growth of lactic acid bacteria, Lactobacillus casei YIT 9018(wild type strain with prophage) or L. casei HYM 1213 (prophage cured strain) was infected with various concentrations of phage ${phi}$ FSV (called Lac S21) or wild type virulent phage (called Lac J-1). When L. casei YIT 9018 was infected with Lac S21 under the concentration of $6.0{ imes}10^6$ pfu/ml, the growth and lactic acid production of the strain were normal and the number of phages decreased. But L. casei HYM 1213 was susceptible to Lac S21. Regardless of the concentration of the phage infection, the number of phages increased rapidly into $10^9$ to $10^{10}$ pfu/ml at 2 day cultures and was maintained $10^7$ to $10^9$pfu/ml phage until 6 day cultures. The lactic acid production of L. casei HYM 1213 infected with Lac S21 was abnormal. Therefore, phage ${phi}$ FSV had an evil effect on growth of L. casei HYM 1213, but not L. casei YIT 9018. On the other hand Lac J-1 caused abnormal fermentation to either L. casei YIT 9018 or L. casei HYM 1213.