Carnobacteriocins BM1 and B2 are thermostable class II bacteriocins produced by Carnobacterium piscicola LV17B. These bacteriocins were purified by a three-step procedure that included hydrophobic interaction, size exclusion, and reversed-phase high performance liquid chromatography. The purified peptides and fragments derived by enzymatic digestion were analyzed by Edman degradation, amino acid analysis, and mass spectrometry. An oxidized form of carnobacteriocin BM1 (carnobacteriocin B1) was also purified and characterized. Probes synthesized using information from the N-terminal amino acid sequences for the purified bacteriocins were used to locate structural genes for the carnobacteriocins. A 1.9-kilobase (kb) HindIII fragment from a 61-kb plasmid (pCP40) containing the carnobacteriocin B2 structural gene and a 4.0-kb EcoRI-PstI genomic fragment containing the carnobacteriocin BM1 structural gene were cloned and fully or partially sequenced, respectively. Expression of the chromosomal bacteriocin and its immunity function requires the presence of the 61-kb plasmid. The results indicate that both bacteriocins are synthesized as prebacteriocins. Post-translational cleavage of an 18-amino acid N-terminal extension at a Gly-Gly (positions -2 and -1) site takes place in each prepeptide to yield the mature 43-amino acid carnobacteriocin BM1 (molecular mass 4524.6) and the mature 48-amino acid carnobacteriocin B2 (molecular mass 4969.9). These two peptides showed significant amino acid homology to each other and with those class II bacteriocins which contain the YGNGV amino acid motif near the N terminus.