Gassericin A, produced by Lactobacillus gasseri LA39, is a hydrophobic circular bacteriocin. The DNA region surrounding the gassericin A structural gene, gaaA, was sequenced, and seven open reading frames (ORFs) of 3.5 kbp (gaaBCADITE) were found with possible functions in gassericin A production, secretion, and immunity. The deduced products of the five consecutive ORFs gaaADITE have homology to those of genes involved in butyrivibriocin AR10 production, although the genetic arrangements are different in the two circular bacteriocin genes. Gaal is a small, positively charged hydrophobic peptide of 53 amino acids containing a putative transmembrane segment. Heterologous expression and homologous expression of Gaal in Lactococcus lactis subsp. cremoris MG1363 and L. gasseri JCM1131T, respectively, were studied. Gaal-expressing strains exhibited at least sevenfold-higher resistance to gassericin A than corresponding control strains, indicating that goal encodes an immunity peptide for gassericin A. Comparison of Gaal to peptides with similar characteristics found in the circular bacteriocin gene loci is discussed.