This study aimed to determine the bacterial diversity in human subgingival plaque using culture-independent molecular methods. The researchers analyzed subgingival plaque samples from healthy subjects and those with various periodontal diseases, including refractory periodontitis, adult periodontitis, HIV periodontitis, and acute necrotizing ulcerative gingivitis. They used 16S ribosomal DNA (rDNA) PCR and cloning to identify bacterial species. A total of 2,522 clones were analyzed, with nearly complete sequences of about 1,500 bases obtained for putative new species. Approximately 60% of the clones belonged to 132 known species, while about 40% were novel phylotypes. Known periodontal pathogens such as *P. gingivalis*, *B. forsythus*, and *T. denticola* were identified but were minor components of the plaque. Several novel phylotypes were found in multiple subjects, and some were associated with disease. The study estimated that there are 347 species or phylotypes in subgingival plaque, with an additional 68 unseen species, totaling approximately 415 species. When considering other oral surfaces, the total species diversity in the oral cavity is estimated to be around 500 species. The findings highlight the complexity of the subgingival microbial community and the potential for novel pathogens to contribute to periodontal diseases.This study aimed to determine the bacterial diversity in human subgingival plaque using culture-independent molecular methods. The researchers analyzed subgingival plaque samples from healthy subjects and those with various periodontal diseases, including refractory periodontitis, adult periodontitis, HIV periodontitis, and acute necrotizing ulcerative gingivitis. They used 16S ribosomal DNA (rDNA) PCR and cloning to identify bacterial species. A total of 2,522 clones were analyzed, with nearly complete sequences of about 1,500 bases obtained for putative new species. Approximately 60% of the clones belonged to 132 known species, while about 40% were novel phylotypes. Known periodontal pathogens such as *P. gingivalis*, *B. forsythus*, and *T. denticola* were identified but were minor components of the plaque. Several novel phylotypes were found in multiple subjects, and some were associated with disease. The study estimated that there are 347 species or phylotypes in subgingival plaque, with an additional 68 unseen species, totaling approximately 415 species. When considering other oral surfaces, the total species diversity in the oral cavity is estimated to be around 500 species. The findings highlight the complexity of the subgingival microbial community and the potential for novel pathogens to contribute to periodontal diseases.