Dear Editor,Studies of the coevolutionary dynamics between Pseudomonas fluorescens SBW25 and bacteriophage φ2 can explore host resistance and parasite infectivity with applications in the ecological and therapeutic fields.Coevolutionary dynamics determine the efficacy of phage-based therapy.In the study described here,bacterial resistance and phage infectivity fluctuated with culture time,perhaps resulting from random mutation and temporal adaptation,which reminds us of the necessity to consider evolutionary mechanisms when applying phage to treat bacterial infections.