To assess the frequency of extracellular superoxide (O2) production by enterococci, multiple species were surveyed in a whole organism assay for their ability to reduce ferricytochrome c in a superoxide dismutase-inhibitable fashion. For stool and clinical enterococcal isolates and 12 type strains, only Enterococcus faecalis (87/91 isolates and ATCC 19443), Enterococcus faecium (5/13 isolates), Enterococcus casselijlavus (ATCC 25788), and Enterococcus gallinarum (ATCC 35038) produced extracellular O2. Among 106 strains comprising 13 species of enteric gram-negative bacilli and gram-positive cocci, only Lactococcus lactis subspecies lactis produced extracellular O2, Mean (±SE) rates of extracellular O2 production in vitro by E. faecalis for isolates associated with bacteremia and endocarditis and for isolates from stool were 2.4 ± 0.2, 1.9 ± 0.2, and 1.5 ± 0.3 nmol of O2 min−1 109 cfu−1, respectively (P = .025, analysis of variance), suggesting an association between invasiveness and extracellular O2 production.