The purpose of this investigation was to evaluate of the specificity of Fusobacterium nucleatum subspecies-specific DNA probes using dot blot hybridization. To confirm whether the clinical isolates were F. nucleatum or not, 16S rDNA of them were cloned and sequenced. The sequencing data were used in homology search with database of GenBank. When the homology was above 98% compared with the nucleotide sequence of a certain bacteria, it was judged as the same species with the bacteria. 23 strains of F. nucleatum were isolates from subgingival plaque of periodontitis patient. The clinical isolates of F. nucleatum were classified into 10 groups using phylogenetic analysis of 16S rDNA sequence. F. nucleatum subspecies nucleatum-specific DNA probe Fu4(1.3 kb) reacted with genomic DNAs from 8 type strains of F. nucleatum and it reacted strongly with those from 8 clinical isolates. The Fp4(0.8 kb) reacted with F. nucleatum subsp. polymorphum ATCC 10953 and one clinical isolates. Fv35(1.9 kb) and Fs17(8.2 kb) probes reacted with genomic DNAs from F. nucleatum subsp. vincentii ATCC 49256 and F. nucleatum subsp. fusiform ATCC 51190, respectively. Our results showed that it is not enough to evaluate the specificity of F. nucleatum subspecies-specific DNA probes with only dot blot hybridization. Therefore, Southern blot analysis will be necessary to confirm the specificity of F. nucleatum subspecies-specific DNA probes.