A 58-years-old male with history of previous aortic prosthetic endocarditis caused by Enterococcus faecalis was admitted due to dyspnoea and fever. The two sets of blood cultures were positive for Neisseria bacilliformis. Transoesophageal echocardiography did not demonstrate endocarditis signs, but PET-CT scan showed active infection signs in the valvular aortic tube and possible infection in the aortic prosthetic valve. A six-week course of ampicillin was prescribed; gentamicin was added during the first two weeks. The patient continued a favourable clinical course. This is the 3rd described case of N. bacilliformis endocarditis and the first one in a prosthetic valve.