Phage attack has always been a major problem in industrial fermentation, especially in the dairy industry. In a monitoring test, a total of five kinds of phage against Streptococcus thermophilus were detected in three yogurt factories located from Kanto to Kansai area of Japan. These phages were found to be derived from a same type of phage harboring a restriction-modification (R/M) system. The occurrence of these phages is possibly caused by the horizontal transmission between factories and the adaption to different starter strains used in each plants via the R/M system. Strategies for prevention of phage contamination in yogurt plants were discussed.