A strain of S. typhosa was isolated during routine examination of the carrier from which Panama 58 was originally isolated. This strain was avirulent for mice. A dosage of 1 billion organisms in saline failed to kill any of 20 mice. Infected with mucin the LD50 dose was 600,000 times that required by the standard strain 58. Except for lack of the "d" factor, the avirulent strain was identical with the virulent strain.