Upon infection of Escherichia coli with the temperature-sensitive bacteriophage T4 mutant, L141, at 43 degrees, DNA polymerase activity is induced, yet there is a marked decrease in DNA synthesis. Temperature shift experiments of infected cultures show dramatic changes in the rate of DNA synthesis, while the polymerase activity in extracts remains constant. Unlike most temperature-sensitive systems in which the lesion can be traced to an unstable enzyme, L141 provides a unique example in which the catalytic properties of the enzyme and the change in the nature of the template caused by the increased temperature combine to reduce the rate of DNA synthesis.