Bacterial DNA is synthesized in Escherichia coli strain K12 lysogenic for bacteriophage λ after the phage is induced. Induction of λ phage carrying a mutation in cistron N or Q stimulates synthesis of bacterial DNA in the region between the gal operon and the λ attachment site within 20 minutes after induction. Little or no increase of the specific bacterial DNA is seen upon induction of λ mutants defective in cistron O or P. These results suggest that the λ enzymes for the replication of the phage genome (known to be specified by O and P cistrons) copy “gratuitously” the bacterial chromosome beyond the λ attachment site in the direction of the gal operon. The observed replication is suggested to be the cause of escape synthesis of enzymes of the gal operon which has been shown to occur following the prophage induction (Buttin, Jacob & Monod, 1960; Yarmolinsky & Wiesmeyer, 1960).