Amber mutants of bacteriophage can be reverted by ionizing radiation to pseudo wild type particles, i.e. particles able to propagate in a suppressorless host. Since the DNA of phi X 174 is single stranded mutation cannot be due to errors in repair of radiation damage by the excision repair or by the recombination repair system of the host cell, unless local base pairing occurs accidentally at the site of the mutation studied. When the circular single stranded phi X 174 DNA enters the host cell, it is converted by host enzymes into a double stranded RF, which is subsequently replicated. The results presented show that expression of a radiation induced mutation does not depend on replication of RF.