Ribosomal DNA polymorphism was studied in order to demonstrate intra- and interspecies differentiation of 42 Bifidobacterium strains. DNA from these strains was digested with the endonucleases BamHI, EcoRV, HindIII and PvuII and then analysed by Southern blotting. Ribosomal patterns using a part of an rRNA 23S gene as a probe clearly differentiated the majority of species from each other. Only B. indicum ATCC 25912T and B. infantis ATCC 15697T displayed identical ribosomal patterns, even though they are classified into two different species. Moreover, ribotypes were able to distinguish between strains belonging to the same species. Furthermore, these strains generally showed common bands, except for B. infantis strains and two strains of B. animalis.