The Escherichia coli mutant hfl-1 is lysogenized at very high frequency by bacteriophage lambda. The normal requirement for the lambdacIII gene product in the establishment of repression is not observed in hfl-1 strains. These phenotypic characteristics are specified by a single locus at 82.5 min on the E. coli map in extremely close proximity to the purA gene, cotransduction frequencies ranging from 97 to 100% depending on the particular purA marker used. The lactose operon is shown to function normally in this strain, and there are also no demonstrable differences in ribonucleic acid polymerase activity or cyclic-adenosine monophosphate levels. Alterations in the cell envelope are indicated by a slight rifamycin resistance, which is reversible by pretreating the cells with ethylenediaminetetraacetic acid, and by a resistance to penicillin and a sensitivity to high concentrations of sodium dodecyl sulfate. It is not known whether this change in cell surface is the primary lesion, or a pleiotropic effect of some more basic metabolic shift.