For the purpose of understanding the precise mechanism of staphylococcal infections in human mouth, the author deemed it worthwhile to investigate the possible correlations existing between the staphylococcal strains and some other kinds of organisms usually found in various loci in the mouth. The present paper presents some of the results of an epidemiological study on a total of 845 strains of staphylococci (106 of aureus and 729 of epidermidis strains) which were isolated from the nose, throat, saliva and tooth plagque materials of healthy individuals, and a total of 609 strains of staphylococci (180 of aureus and 429 of epidermidis strains) which were isolated from a variety of morbid sources in the mouth such as periodontosis, surgical lesions, infected root canals and periapical cystic lesions. The isolated organinsms from these sources were respectively characterized for the biological natures by routine methods and the phage typing was conducted on the strains of aureus species to secure more systematic classification. Distribution of both kinds of organisms from the above loci of healthy and morbid individuals was then determined and results were summarized as follows. 1. Healthy subjects usually carried an abundant population of Staphylococcus epidermidis and a small number of Staphylococcus aureus in a parasitic state in their nose, throat and salivary fluid. 2. Morbid materials from periodontosis and surgical lesions showed the presence of Staphylococcus epidermidis, accompanied by a small number of Staphylococcus aureus which was apparently responsible for the infection. 3. In those closed loci in the mouth such as infected root canal and periapical cyst, both Staphylococcus aureus and Staphylococcus epidermidis were present with generally epual abundance, but, in a few instances, the former organisms exceeded the latter in number. 4. The phage typing study conducted on two groups of aureus strains of Staphylococcus isolated from nose and throat region and from salivary fluid revealed that both groups of organisms were identical in the phage typing pattern showing the occurrence of I-type, II-type, and the mixed type of the former two. 5. The phage typing patterns of the isoltes from the infected root canal and periapical lesion were common in that the organisms were of I-III-mixed type. 6. According to the phage typing study on staphylococcal organisms isolated from the nose, throat and salivary flind of healthy individuals, I-type occurred most frequently, followed by III-type and I-III-mixed type in order, while with those from the morbid sources in the mouth, I-III-mixed type was most predominant, followed by III-and I-type in order. 7. Study on the distribution of staphylococcal strains isolated separately from the morbid sources of the mouth and from the mose, throat and salivary fluid of the same individual, revealed that a relatively small number of organisms were present in a parasitic state to cause infections in the former sources, and a similar number of organisms were present in the latter sources, but purely in a parasitic state. 8. Study on the distribution of individual phage types of organisms in both morbid sources and in the nose, throat and salivary fluid, revealed that both sources yielded the identical pattern of phage type distribution, I-type, III-type and I-III-mixed type being all present. The above study would seem to suggest that the organisms usually present in the nose and throat can readily migrate into the salivary fluid and can become the constant source for initiation of the primary and secondary infections in oral tissues and organs.