The nature of the genetic determinant of smegmatocin, a bacteriocin produced by Mycobacterium smegmatis was investigated by testing the loss of bacteriocin-producing ability during treatments with various curing agents and by cultivation at an elevated temperature. As none of these treatments produced any loss of smegmatocin-producing capacity of the bacterial strain, the genetic determinant of smegmatocin is probably not of extrachromosomal DNA but rather is located on the host chromosome.