Oligonucleotide primers were designed from the DNA sequence of the aroA region from Lactococcus lactis and these were used to amplify regions adjacent to the aroA gene. The amplified fragments were cloned to produce a suicide plasmid vector for chromosomal integration. Transformation of L. lactis resulted in a single cross-over homologous recombination event and subsequent excision of the plasmid generated a strain lacking the aroA gene. Growth characteristics indicated that the mutant strain was deficient ire aroA.