The phage insensitivity gene of lactococcal plasmid pCI829 which encodes an abortive infection defense mechanism (Abi) was inserted into the Lactococcus lactis subsp. lactis CH919 chromosome by utilizing the integration plasmid pCI194, which contains 4.2 kb of homology with the conjugative transposon Tn919. Chloramphenicol-resistant transformants expressed phage insensitivity to the prolate-headed phage c2 and the small isometric-headed phage 712, and hybridization analysis indicated that transformants contained pCI194 integrated in single copy. The level of phage insensitivity expressed by the transformants was reduced from that observed when the abi gene was located on a replicating plasmid, as determined by plaque assay and burst size analysis. Amplification of the integrated structure after growth in increased concentrations of chloramphenicol resulted in an increase in the expression of phage insensitivity. Hybridization analysis revealed that while pCI194 was stably maintained in an integrated state over 100 generations in the absence of selective pressure, the ability to express phage insensitivity was lost. Hybridization analysis also revealed that DNA flanking the abi gene contains homology to the CH919 chromosome.