For many bacteriophages (phages), the proteins responsible for host lysis and virion morphogenesis are expressed from the same polycistronic transcript. Such an expression pattern can potentially have a pleiotropic effect on the assembly rate and lysis time, thus affecting phage fitness. To study the effects of late promoter activity on phage life history traits and fitness, we constructed a series of isogenic phage lambda strains that differ only in their late promoter pR' sequences. The resulting late promoter activities ranged from 6 to 100% of the wild type's. The lysis times, burst sizes, and relative fitness were empirically determined for these strains. Our results showed that the lysis time is more sensitive than the assembly rate to variation in pR' activity. However, except for the strain with the lowest activity, the relative fitnesses of all the other strains are not significantly different from each other. Ad hoc models describing the effects of the late promoter activity on lysis time and assembly rate were constructed. The expected phage burst size and fitness curve were predicted from these models. Evolution of the late promoter activity was discussed in the context of phage life history trait evolution.