The development of molecular biology has opened the way to new approaches to bacterial identification and typing. Nucleic acids carry the information encoding the bacterial diversity. They can be sequenced easily and, even when undetermined, their sequences can be quickly compared by molecular hybridization. In addition, short sequences on double-stranded DNA are recognized by restriction endonucleases as cleavage sites. The number and position of these endonuclease-specific restriction sites on a DNA molecule determine the number and the sizes of the fragments generated by cleavage. The development of agarose gel electrophoresis for the separation of DNA fragments allows the comparison of restriction patterns of plasmid (1 ) and chromosomal DNA (2 ).