A bacteriophage T5 mutant has been isolated that is completely deficient in the induction of deoxynucleoside 5'-monophosphatase activity during infection of Escherichia coli F. The mutant bacteriophage has been shown to be deficient in the excretion of the final products of DNA degradation during infection of E. coli F, and about 30% of the host DNA's thymine residues were reinocorporated into phage DNA. During infection with this mutant, host DNA degradation to trichloroacetic acid-soluble products was normal, host DNA synthesis was shut off normally, and second-step transfer was not delayed. However, induction of early phage enzymes and production of DNA and phage were delayed by 5 to 15 min but eventually reached normal levels. The mutant's phenotype strongly suggests that the enzyme's role is to act at the final stage in the T5-induced system of host DNA degradation by hydrolyzing deoxynucleoside 5'-monophosphates to deoxynucleosides and free phosphate; failure to do this may delay expression of the second-step-transfer DNA.