BACKGROUND Capnocytophaga canimorsus is a gram-negative bacterium and an oral commensal in dogs and cats, but occasionally causes serious infections in humans. Septicemia is one of the most fulminant forms, but diagnosis of C. canimorsus infection is often difficult mainly because of its very slow growth. C. canimorsus infective endocarditis (IE) is rare and is poorly understood. Since quite a few strains produce β-lactamase, antimicrobial susceptibility is pivotal information for adequate treatment. We herein report a case with C. canimorsus IE and the results of drug susceptibility test. METHODS A 46-year-old man had a dog bite in his left hand 3 months previously. The patient was referred to our hospital for fever (body temperature > 38 °C), visual disturbance, and dyspnea. Echocardiography showed aortic valve regurgitation and vegetation on the leaflets. IE was diagnosed, and we initially administered cefazolin and gentamycin assuming frequently encountered microorganisms and the patient underwent aortic valve replacement. C. canimorsus was detected in the aortic valve lesion and blood cultures. It was also identified by 16S ribosome DNA sequencing. Ceftriaxone were started and continued because disk diffusion test revealed the isolate was negative for β-lactamase and this case had cerebral symptoms. The patient successfully completed antibiotic treatment following surgery. CONCLUSIONS We diagnosed C. canimorsus sepsis and IE by extended-period blood cultures and 16S ribosome DNA sequencing by polymerase chain reaction, and successfully identified its drug susceptibility.