A non‐lysogenic strain ofPseudomonas aeruginosamaintained its phage sensitivity pattern and serogroup specificity unchanged for 10 weeks in ex‐germfree, mono‐contaminated rats before infection with phage. After infection with phage, phage conversions of serogroup specificity and lysotype were observed. With the same combination of bacterial and phage strains, the same serogroup was obtainedin vitroandin vivo. In vitroconversion occurred also to serogroups that were not detectedin vivo.Upon lyophilization, converted bacterial clones from thein vivoexperiment lost their phage and simultaneously reverted to the original phage type and serogroup. These findings may have implications for the understanding of the degree of stability in epidemiological typing results forP. aerug