The prevalence and quantity of mycoplasmata in the gingival sulcus area of periodontally diseased and periodontally healthy subjects was tested. Mycoplasma salivarium was the predominant sulcular species and occurred in a significantly higher percentage of diseased subjects than healthy subjects (p<.0005) (86.7 % vs 31.8 %). When mycoplasmata were present in the sulcus their concentration was usually 106–107 per gram of fluid whereas the concentration in saliva was approximately 100 to 1000 fold less. Therefore M. salivarium appears to be primarily a sulcular organism. M. salivarium may be part of the complex plaque which appears to produce gingivitis.