DNA of 33 strains of Streptococcus thermophilus (7 isolated from natural whey cultures utilized as starter, 25 from commercial yogurts and 1 reference strain) was analysed by restriction endonuclease digestion and both conventional and pulsed-field agarose gel electrophoresis. The restriction patterns after digestion of total DNA with HindIII, EcoRI and BamHI enabled 10 different groups to be distinguished by conventional electrophoresis. Digesting genomic DNA with SmaI or NotI and separating it by pulsed-field gel electrophoresis, the 33 strains could be classified into 5 groups. The same result was obtained after digestion with HaeIII and conventional electrophoresis. The 33 isolates could be divided into 13 groups when diversities among patterns obtained after both the first and second method were considered. A combination of the two techniques turned out to be the most reliable methodological approach for characterizing strains of S. thermophilus for scientific, industrial and legal purposes.