ABSTRACT Following infection of E. coli B with ligase-deficient rII bacteriophage T4D recombination between linked markers is increased 4.2 fold and heterozygote frequency increased 2.3 fold. In such infection recombination occurs at a rapid rate for an extended period. This is in contrast to the time course of recombination observed in wild-type, lysis-inhibited, or lysis-defective (gene t defective) infection. In all of these cases recombination under standard cross conditions occurs early in the vegetative cycle. The increased recombination in ligase-deficient rII infection is reduced in a bacterial strain which produces greater than normal levels of host ligase. These results indicate that ligase has a crucial role not only in the replication of DNA but also in recombination. The level of ligase may determine whether DNA replication occurs with or without concomitant recombination.