Streptococcus thermophilus 347 isolated from yogurt produces a bacteriocin referred as thermophilin 347. The bacteriocin was evidenced in the neutralized, filtered and catalase treated culture supernatant fluid of the producer strain. After partial purification, thermophilin 347 exhibited a bactericidal effect against Listeria monocytogenes and several closely related lactic acid bacteria. The activity of thermophilin 347 was lost after protease treatment but was maintained after heating at 100 degrees C for 1 h; after autoclaving at 121 degrees C for 15 min the activity was reduced by 50%. SDS-PAGE of partially purified thermophilin 347 was used to detect bacteriocin activity corresponding to an apparent molecular mass between 2.5 and 6.2 KDa.