Abstract: 4 bacteriophage strains of yogurt were isolated from a fermentation tank in the plant of Bright Dairy & Food Co., Ltd. By electron microscopy observation, they appeared like tadpole. The burst size of No. 1 bacteriophage was 549, and its latent period was less than 15min. The environment of both pH4～6 and 0.04% Ca2+ contributed to their growth. But they were deactivated at 60℃ for 30min. The burst size of No. 2 bacteriophage is 681, and its latent period was less than 15min. The environment of both pH4～8 and 0.04% Ca2+ contributed to their growth. But they were deactivated at 50℃ for 40min or 60℃ for 20min. The burst size of No. 3 bacteriophage was 424, and its latent period was less than 15min. The environment of both pH8～10 and 0.06% Ca2+ contributed to their growth. But they were deactivated at 60℃ for 10min. The burst size of No. 4 bacteriophage was 506, and its latent period was less than 15min. The environment of both pH7～8 and 0.04% Ca2+ contributed to their growth. But they were completely deactivated at 60℃ for 40min.

Key words: Yogurt, Bacteriophage, Streptococcus thermophilus, Lactobacillus bulgaricus, Isolation, Biological charac- teristics