Many antagonistic interactions between hosts and their parasitesresult in coevolution. Although coevolution can drive diversity andspecificity within species, it is not known whether coevolutionarydynamics differ among functionally similar species. We presentevidence of coevolution within simple communities ofPseudomonasaeruginosaPAO1 and a panel of bacteriophages. Pathogen iden-tity affected coevolutionary dynamics. For five of six phages tested,time-shift assays revealed temporal peaks in bacterial resistance andphage infectivity, consistent with frequency-dependent selection(Red Queen dynamics). Two of the six phages also imposed addi-tional directional selection, resulting in strongly increased resistanceranges over the entire length of the experiment (ca. 60 generations).Cross-resistance to these two phages was very high, independent ofthe coevolutionary history of the bacteria. We suggest that coevo-lutionary dynamics are associated with the nature of the receptorused by the phage for infection. Our results shed light on thecoevolutionary process in simple communities and have practicalapplication in the control of bacterial pathogens through the evo-lutionary training of phages, increasing their virulence and effi-cacy as therapeutics or disinfectants.