Fifty isolates of Rothia dentocariosa from diverse clinical sources were characterized by 28 separate tests. An attempt was made to select practical tests that could be completed in a minimal length of time. Rothia is also compared with Actinomyces and Nocardia with which it is often confused. Of the isolates 100% were positive in the following reactions: catalase production, nitrate and nitrite reduction, esculin hydrolysis, and acid production from glucose, sucrose, maltose, salicin, and glycerol. The importance of recognizing this organism is based on the fact that it is frequently isolated from human clinical materials and must be differentiated from morphologically similar organisms of the genera Actinomyces and Nocardia, which contain pathogenic members.