The UV radiation survival curve of exponentially growing cultures of Escherichia coli B/r WP2 trpE65 was modified by pretreatment for short incubation periods (up to 20 min) with chloramphenicol such that an extended exponential section of intermediate slope appeared between the shoulder and the final exponential slope. Surges of mutation to tryptophan independence occurred with each increase in slope of the survival curve. These surges were separated by extended sections of little mutation. Nalidixic acid prevented both the changes in survival and mutation. Mutation curves obtained with overnight cultures had three extended sections of little mutation alternating with sections of high mutation. Reincubation for 60 min in fresh medium reduced or eliminated the low-response sections. These reappeared after 80 to 90 min, when DNA had doubled in the culture and before the initial synchronous cell divisions had occurred. Nalidixic acid prevented this reappearance.