Abstract: Using soybean as the protein basis, yoghurt was made by inoculating FJAT-7928(Streptococcus thermophilus)and FJAT-7929(Lactobacillus bulgaricus).The lactic acid bacteria population, acidity and pH of the yoghurt stored at 4℃ were determined.The result showed that the soy yoghurt prepared by the co-fermentation had a maximum number of the bacteria of 3.5×109 cfu·mL-1, which was more than those in the soy yoghurt inoculated with either FJAT-7928 or FJAT-7929 alone.After 30 days of the cold storage, the acidity of the co-fermented yoghurt was 109.0°T, which was in between 114.0°T for FJAT-7929 yoghurt and 60.0°T for FJAT-7928 yoghurt.The final pH of the co-fermented yoghurt was 3.58, which was lower than 3.65 for FJAT-7929 yoghurt or 4.09 for FJAT-7928 yoghurt.At 4℃, the Lactobacillus population, acidity and pH of all three yoghurt products remained constant after 30 days in storage.