This report describes what is believed to be the first case of mixed Pseudomonas and Serratia endocarditis, of probable nosocomial etiology, with involvement of all four heart valves in a 56 year old nonaddicted patient. Although both organisms were recovered in culture, infection and tissue invasion were documented by light and electron microscopy. The clinical course in this patient differed from more typical patterns of Pseudomonas or Serratia endocarditis that have been observed as complications of narcotic addiction or compromised cardiac status. Our patient had the rare occurrence of endocarditis with two organisms and four-valve involvement. Clinically, however, this presented as a right-sided endocarditis and behaved as though only a single organism were present.