Whole genomic and randomly‐cloned DNA probes for two fastidious periodontal pathogens,Porphyromonas gingivalisandBacteroides forsythuswere labeled with digoxigenin and detected by a colorimetric method. The specificity and sensitivity of the whole genomic and cloned probes were compared. The cloned probes were highly specific compared to the whole genomic probes. A significant degree of cross‐reactivity withBacteroidesspecies.Capnocytophagasp. andPrevotellasp. was observed with the whole genomic probes. The cloned probes were less sensitive than the whole genomic probes and required at least 106 target. cells or a mininium of 10 ng of target DNA to be detected during hybridization. Although a ten‐fold increase in sensitivity was obtained with the whole genomic probes, cross‐hybridization to closely related species limits their reliability in identifying target bacteria in subgingival plaque