Mu ligts2 mutants, defective for development and integration, show a high killing effect on the infected host. A number of survivors to Mu ligts2 infection were analyzed; they are characterized by nonpermissivity for both development and lysogenization of bacteriophage Mu. Bacteriophages D108 and P1 are also inhibited in these strains as is transposon Tn9. The corresponding mutation site was mapped at 82 min and identified with the Escherichia coli gyrB site.