Abstract:Streptococcus thermophilus and Lactobacillus delbrueckii subsp.bulgaricus are the main bacteria that make up the yoghurt starter.These cooperative symbiotic relationships endow the starter strains with fast acid production，high viscosity，rich active substances and other excellent properties，and also have an important impact on the quality of yogurt.The cooperative symbiotic relationships between the two bacteria from the aspects of nutrition supply，resistance to environmental stress and improvement of yogurt quality were discussed in this paper.These relationships provided theoretical guidance for improving the vigor of starter strains and yogurt quality.