Periodontitis is believed to be caused by bacteria which inhabit periodontal pockets. The identification of these periodontal pathogens by currently available methods requires considerable time and expertise. In this study, we have used a polymerase chain reaction (PCR) assay that is quick, relatively simple, and can detect low numbers of a putative periodontal pathogen. Primers specific for the fimbrial gene of Porphyromonas gingivalis were selected from the published sequence and used for amplification of a 131-basepair sequence of genomic DNA. The PCR detected as few as 100 P. gingivalis cells obtained from pure cultures. Three other bacteria (Actinobacillus actinomycetemcomitans, Prevotella intermedia, and Capnocytophaga gingivalis) that are also putative periodontal pathogens yielded no PCR product at any of the cell concentrations used. This assay was also used for detection of P. gingivalis in subgingival plaque. Five of 13 subgingival bacterial plaque samples obtained from four advanced adult periodontitis patients and two samples from a prepubescent child with advanced periodontitis contained P. gingivalis. The protocol developed is relatively simple and can be completed within four hours of the time of sample acquisition.