A total of 142 presumptive lactococci were isolated from 40 raw milk samples and screened for bacteriocin production. Four presumptive lactococci isolates (YB23, YB42, YB98 and YB112) exhibited antimicrobial activity against indicator strains. Antimicrobial activity and protease sensitivity assays showed that a strain, YB23, produced bacteriocin that inhibited a broad spectrum of Gram-positive bacteria, and was selected for further analyses. It was identified as Lactococcus lactis subsp. lactis, phenotypically by the A PI system and genotypically by 16S rDNA homology. YB23 was tested for determination of antimicrobial activity against indicator strains including known bacteriocin producing lactococci and it showed no activity against nisin producer Lactococcus lactis SIK83. Different pH and heat treatments showed that the bacteriocin produced by L. lactis subsp. lactis YB23 exhibited a similar behavior with nisin produced by L. lactis SJK83. The bacteriocin produced by YB23 strain was characterized as nisin by PCR analysis of its genomic DNA. Lactic acid production and proteolytic activity in strain YB23 were determined as 0.38% and 25.3 mu g Tyr/mL, respectively.