A high-frequency transformation system was developed for Lactobacillus reuteri with the electroporation technique. With this method, transformation frequencies of 10(7) transformants per mu-g DNA were routinely obtained with the plasmid pLUL631 and its derivatives. pLUL631, a native L. reuteri plasmid containing an erythromycin resistance determinant, was studied mainly with regard to replicon size and stability. The minimal replicon of the plasmid was shown to be located on a 2.4-3.1 kilobase pair fragment. pLUL631 was stably maintained in several L. reuteri strains, but a deletion derivative, pLUL634, was unstable during growth under nonselective conditions. A hybrid plasmid, pLUL200, derived from the broad-host-range plasmid pVS1 and a part of the pLUL631 replicon, was shown to have higher copy number than both pLUL631 and pLUL634. The replicon of pLUL631 is suggested to be a suitable part of a food grade vector for L. reuteri.