Abstract: To obtain yogurt production strains with functions of phage resistant and good fermentation performance,protoplast fusion between single inactivated Lactobacillus bulgaricus and bacteriophage-resistant mutant of Streptococcus thermophilus was performed by the induction of PEG6000 and the performances of fusant were studied. Finally,three lactic acid bacteria fusants with functions of phage resistant and good fermentation performance were selected from 20 fusants that were obtained through fusion. The fusion rate reached 1.85 ×10-6after 2 min of infusion at 40 ℃ in the PEG6000. The performances of the fusant were superior at producing acid,solidifying milk,producing good smell and resisting phage,so it was applicable to yoghurt production.