Main part of eukaryotic genomes is build of unique sequences coding proteins and RNAs, but they contain as well numerous repeats interspersed with single-copy fragments. Existence of repetitive sequences were also demonstrated in in prokaryotic genomes. They are found in different species of Gram-negative and Gram-positive bacteria. Interspersed repetitive sequence elements called REP and ERIC sequences are present in different species of Enterobacteriaceae family, including Escherichia coli and Salmonella typhimurium. Their functions are not completely clear, probably they play important role in regulation of gene expression. Nevertheless, REP and ERIC elements are widely use in identification and genetic analysis of bacteria. For example, using rep-PCR technique it is possible to descriminate between closly related serovars of the same species, which enables to analyse phylogenetic and epidemiological relations among them.