A typical gram-positive pleomorphic aerobe is Rothia dentocariosa. However, there exist many organisms having similar characteristics (Rothia-like organisms). The purpose of this study is to isolate Rothia dentocariosa and Rothia-like organisms from the healthy and diseased sites of the oral cavity and to clarify the characteristics of those organisms. A total of 342 strains were isolated in this study, and 188 strains were identified as gram-positive pleomorphic rods. Seventy-seven typical strains of 188 isolates were examined physiologically and biochemically, and they were classified as follows: 1) 20 strains of A. viscosus; 2) 3 strains of A. naeslundii; 3) 26 strains of Rothia dentocariosa; 4) 2 strains of Bacterionema matruchotii; 5) 15 strains of Corynebacterium; 6) 7 strains of Nocardia; and 4 unknown strains. The strains of Rothia dentocariosa were mainly isolated from the dental plaque (24 of 26 isolates), and they had the ability to adhere on to the glass surface, and 5 of 26 strains of Rothia dentocariosa showed the ability of gelatin liquefaction.