Gassericin A, a bacteriocin that was produced by Lactobacillus gasseri LA39, was treated with lysylendopeptidase and 3-bromo-3-methyl-2-(2-nitrophenyl-mercapto)-3H-indole. The fragments were recovered by SDS-PAGE and sequenced. All amino acids of gassericin A were distributed by sequence analysis and the bacteriocin did not contain any modified amino acids. The amino acid sequence of gassericin A completely coincided with that found through the cloning of the structural gene. Gassericin A was shown to be a cyclic bacteriocin (class II) which is bound at the N- and C-terminal ends. This is the first example of a cyclic bacteriocin from lactobacilli lactic acid bacteria.