A new case of Neisseria elongata ssp. nitroreducens bacteremia and endocarditis in a 74-year-old woman who had undergone aortic valve replacement in 1992 is reported in detail. N. elongata ssp. nitroreducens differs from the other subspecies of N. elongata in the additional reduction of nitrate without gas formation. Like most Neisseria spp. except Neisseria meningitidis and Neisseria gonorrhoeae, this N. elongata ssp. nitroreducens is usually classified in the group of "non-pathogenic" Neisseria spp. This case report indicates that the presence of subspecies of this group is significant when isolated from normally sterile sites and can cause severe disease in susceptible individuals.