Bacillus subtilis Marburg is nonpermissive for the multiplication of bacteriophages SP10 and phi NR2. A permissive mutant was derived from the Marburg strain, and the genetic determinants of non-permissiveness were analyzed by PBS1 transduction. The simultaneous presence of two genes as mutant alleles, nonA and nonB, was necessary for permissiveness. The gene nonA is linked very closely to rfm (cotransfer: 95%); nonB is located between dal and purB (cotransfer of nonB and purB6 : 48%). The genetic determinant of host-specific restriction intrinsic to the Marburg strain (hsrM) was found to be identical or very closely linked to nonB. The segregation on nonB and hsrM has never been observed in the course of transduction analysis. The mutation, hsrM1, diminishes the restriction activity, but not the host-controlled modification.