The occurrence of Rocky Mountain spotted fever on the Isthmus of Panama was established for the first time in 1950 when a highly virulent strain of Rickettsia rickettsii was isolated by the author from the blood of a 26 year old Panamanian farmer who died February 8 of that year, shortly after admission to the hospital. The patient evidently had contracted his infection near the town of Ollas Arriba about seven miles north of Capira in the Province of Panama. Since then similarly virulent strains of R. rickettsii have been isolated from two additional cases, hospitalized in May 1950 and February 1951, both male, aged 13 and 26 years respectively, who were employed in agricultural work on farms located in the same area as the previous case. The 13-year old boy died. The 26-year old man survived after a severe and protracted illness. His blood showed complement-fixing antibodies against Rocky Mountain spotted fever antigen in high titer during convalescence.