Kingella species are rarely reported as a cause of endocarditis. We report two cases ofK. denitrificans endocarditis occurring in patients with prosthetic valves.K. denitrificans has not been previously reported to cause prosthetic valve endocarditis. Both patients may have developed bacteremia from altered upper respiratory or oral mucosa with subsequent prosthetic valve infection. Treatment with penicillin G and streptomycin was successful in the first patient, whereas ampicillin alone resulted in clinical cure in the second case. Neither patient required further surgery. In vitro antibiotic susceptibility testing indicated that both organisms were highly sensitive to cefotaxime and ampicillin.