The 16S-23S rDNA internal transcribed spacer (ITS) regions of all currently defined Fusobacterium species and related taxa such as Leptotrichia buccalis, Sebaldella termitidis and Streptobacillus moniliformans, were analysed to examine inter- and intraspecies as well as subspecies relationships. For the ITS-amplification, a new eubacterial universal primer pair was designed and used. The majority of the Fusobacterium strains, along with L. buccalis showed one major, and two to three weaker, distinct bands (short and long versions) with lengths of 800-830 bp and 1000-1100 bp. Nevertheless, six other patterns were also found within the genus Fusobacterium, demonstrating its heterogeneity. The ITS region was sequenced and found to consist both of conserved motifs, which functioned as a framework for alignment, and of variable sites, which provided high phylogenetic resolution. Analyses of the ITS-DNA sequences and ITS relative length (short version) allowed species and subspecies differentiation in most cases. The results confirmed the strikingly distant relationship between Fusobacterium prausnitzii and the genus Fusobacterium. Fusobacterium nucleatum subspecies, along with Fusobacterium naviforme, Fusobacterium simiae and Fusobacterium periodonticum, formed a cluster with an inherently high potential for diversification. Other clusters were formed by Fusobacterium necrophorum subspecies with Fusobacterium gonidaformans and by Fusobacterium varium with Fusobacterium mortiferum and Fusobacterium ulcerans. Fusobacterium russii as well as Fusobacterium perfoetens formed separate branches. Fusobacterium necrophorum subspp. necrophorum and funduliforme on the one hand, and Fusobacterium varium and Fusobacterium mortiferum on the other, were found to be very similar, even at the high-resolution ITS level.