AbstractInfection ofEscherichia coli: with T4 bacteriophage causes the appearance of a new valyl‐tRNA synthetase activity associated with a molecule that, compared to the host enzyme, exhibits a greater resistance to denaturation by heat or urea, a larger molecular volume, a higher rate of sedimentation in sucrose gradients, a greater net positive charge, and a greater ability to charge yeast tRNA. No evidence has been found for similar changes in synthetase activity for the other amino acids. Appearance of the new activity requires continued protein synthesis and results from a modification of the preexisting host enzyme rather thande novosynthesis of a totally new enzyme. By 20 minutes after infection at 30°C, all of the host enzyme has been converted into the new form. Phage mutants have been isolated that fail to effect a normal conversion. The properties of these mutants suggest that conversion involves the addition, to the host enzyme, of a protein specified by the phage genome. Drastic reduction of phage‐induced activity in one of these mutants does not interfere detectably with phage development in a normal host, suggesting that the presence of the new activity is not essential for normal phage produc