Rothia dentocariosa was proved as a primary pathogen for man in the case of abdominal infection of a 19 year old woman. Clinical findings were typical for actinomycosis and the patient was cured with penicillin following surgery. Actinomycotic granules were proved by direct microscopy in the pus and R. dentocariosa was recovered as a single pathogen according to the microscopy. Two morphologically distinct isolates recovered from primocultures were described. They were studied separately and finally they were recognized as physiologically identical varieties of a single organism. In addition, the two morphological varieties of R. dentocariosa were considered to represent a particular kind of variability of the A-N type. The isolated germ was found to be pathogen for mice. Intraperitoneal infections provoked multiple nodules formation which were grown into the internal organs of experimental animals. The nodules were encapsulated and tended to spread without regard to the natural tissue plan. They contained pus involving microgranules of viable germs. This first report of the natural human infection caused by R. dentocariosa is the precedent for the definite establishing of this aerobic glucose fermenting actinomycete amidst the untraditional potential pathogens of human actinomycosis.