Bacteriophages T2, T4 and T6 (T-even phages) do not make plaques normally when they are plated with a mutant of Escherichia coli K12, W4597, which has been shown to synthesize an abnormally low level of UDPG† due to a defect in UDPG pyrophosphorylase. T-even phages are adsorbed normally by W4597 cells, and induce replication of viral DNA. Infection by T4 of the strain W4597 induces the synthesis of hydroxymethyl cytosine- β -glueosyl transferase but does not evoke the activity of UDPG pyrophosphorylase. Growth of T-even phages on W4597 yields phage particles with deficiently glucosylated DNA which cannot grow on E. coli strains K12 or B but can multiply on strains of Shigella dysenteriae . These results thus indicate that UDPG, the synthesis of which is directed by host bacteria, is the normal and probably the sole glucosyl donor to DNA of T-even phages, and that the glucose moieties of DNA do not contribute genetic specificity to T-even phages but determine the ability of viral DNA to replicate successfully on certain hosts.