Summary Mice, initially immunized with active rickettsiae and resistant to subsequent inoculation, could not withstand invasion of reinoculated rickettsiae into tissues: that is to say, superinfection is possible in experimental Tsutsugamushi disease. Superinfection occurs whether doses of rickettsiae as large as 105 LD50 or as small as 10 LD50 are used. Reinoculated rickettsiae invade immunized tissues, and persist longer than 180 days.