The complete nucleotide sequence of a cryptic plasmid pLA105 from Lactobacillus acidophilus TK8912 was determined. Sequence analysis revealed that pLA105 has a molecular size of 3,220 bp and contains two open reading frames (ORFA and ORFB) encoding polypeptides of 127 and 123 amino acid residues. The ORFA-encoded protein showed a significant homology (75%) to the replication protein of pediococcal plasmid. However, ORFB exhibit no homology to DNA or protein sequences involved in gram-positive bacterial plasmid replication. The region preceding ORFA contains conspicuous sequences of inverted and direct repeats. The plasmid vector containing this region was able to replicate in the Lactobacillus host when provided with ORFs A and B in trans, suggesting that the repeat region contains the origin sequences essential for pLA105 replication. The constructed plasmid vectors based on the pLA105 replicon are structurally and segregationally stable in the Lactobacillus host.