A case of septic arthritis due to Vibrio fetus is presented. This rare human infection occurred in a knee joint with a background of old and recent trauma. Treatment by repeated joint aspirations plus administration of parenteral penicillin and streptomycin had little effect on control of the infection until ampicillin was added to the regimen. After surgical removal of an old corroded screw and washer, recovery was uneventful and return of function was complete.