The occurrence of the dental caries in the Old Slavonic populations from the 9th century from Central Europe i.e. from the Great Moravian period is very frequent. The predispositions for caries forming are in general: personal hygiene, food and daily habits, density, thickness and integrity of dental enamel, secretion of salivary glands and oral bacterial flora. Unfortunately, personal hygiene or daily habits cannot be studied in ancient populations when there are no marks on teeth. The composition of food of Slavs was being studied in many other researches, so firstly during our research a bibliographic search on this topic was made. The density, thickness and integrity of the Slavonic dental enamel will be study later. Our main focus within practical part of our research was on studying the oral bacterial flora. The direct way to do this is using of dental calculus because it contains mineralized bacteria. Scanning electron microscope was used for this purpose. The sample was dry enough so there was no need to adjust it. Bacteria like streptococci, cocci, rods etc. are visible in the pictures. They are in places which were not exposed to external factors like washing or environment in the soil. It is possible to determine only genera but a particular species cannot be differentiated. In our research the existence of bacteria was proved in sample of the dental calculus which is more than thousand years old.