Rifampicin dependent mutants had been isolated as conditional lethal mutants with a lesion affecting transcription. Substituting Tn10 coded tetracycline resistance as a stable marker instead of the unstable rifampicin dependence facilitated mapping of dependence mutations. Three new rid loci were defined in this way. ridB and ridD were on opposite sides of ilv at about 84 min, and ridC was at about 35 min on the Escherichia coli chromosomal linkage map. Two more mutants with the previously described ridA lesion were also found. Growth response to removal of rifampicin was different for ridA, B, or C strains. Double mutants were constructed, and in all cases the more limiting phenotypic response prevailed.