To select bacteriophage-resistant mutants suitable for yoghurt production,the method of double layer agar plate and natural selection were used. Three different phages were isolated from the abnormal fermentation broth of Streptococcus thermophilus St0 which producing yoghourt. Four bacteriophage-resistant mutants( St1,St2,St3,St4) were obtained and proved to be non-lysogen by Mitomycin induction. The resistance and the yoghourt fermentation ability of these mutants were stable after 20 subcultures. The mutant St3 produced total acids up to 110° T after fermentation and the yogurt curd time was as same as the origin strain. When the mutant strain was cultured under higher concentrations phage environment,the bacteria growth was normal,and its acid production,viscosity,hardness,taste were all better than those of original strain. In the meantime,shorter curd and less whey separation were obtained and bacteria concentration was up to 1010CFU/mL after enrichment culture. So it was suitable for industrial production.