A mutant strain of E. coli K12, R2721, differs from its parent strain, S491, in 4 associated phenotypic characters as a result of a single mutation. This strain did not give recombinants with DNA transduced by bacteriophage P1 or bacteriophage Mu, nor transformants after exposure to R factor DNA; lysates of bacteriophage P1 grown on this strain did not appear to contain any transducing particles when tested on normal recipients. The reversion rates, both spontaneous and UV-induced, for 2 auxotrophic markers were reduced. The frequency of revertants was at least 2 orders of magnitude lower in cultures of R2721 than in cultures of S491. Many of the rare revertants for one or other of the auxotrophic markers regained normal reversion frequencies for the other marker and for the capacity to be transduced. In all other respects, recombination in R2721 appeared normal, the frequency of chromosomal mobilization by an F'' factor was unaffected and normal yields of recombinants were obtained from matings with Hfr strains. The only circumstance in which transduction of R2721 was observed was when the capacity to ferment galactose was selected and P1 was grown on a strain carrying .lambda.dgal when, presumably, integration was effected by the phage-coded gene products. The mutation was located on the E. coli chromosome map between tonA and pro and was given the symbol tdi (transduction inhibition). Double mutants, (tdi recA) and (tdi recB), were isolated and show no unexpected properties.