THE evolutionary basis for the integration of phage DNA into bacterial chromosomes (lysogeny1,2) is not understood. A generally accepted explanation is that when the bacterial population is large and growing well, the phages prefer the lytic pathway since they can multiply without elimination of all host bacteria. If conditions deteriorate so that the supply of bacteria becomes limited or if growth is slowed, the phages tend to lysogenise. There have been few studies of the evolutionary mechanisms which may have selected for lysogeny. We now report that in certain growth conditions λ lysogens of Escherichia coli reproduce more rapidly than isogenic, non-lysogenic strains.