Growth of Escherichia coli 15t- in a thymine-containing medium with any of several antibacterial agents (5-aminouracil, mitomycin C, nalidixic acid or hydroxyurea) increased the 6-methyladenine content of the extractable DNA and the DNA methylase activity. Similar changes occurred after transient (45 min.) thymine deprivation but not with other antibacterial agents such as penicillin, phenylethyl alcohol, novobiocin. Associated with the increases in 6-methyladenine content and DNA methylase activity was the induction of a defective bacteriophage. Similar effects were noted in the wild-type E. coli 15.