TwoPseudomonas aeruginosastrains, one monoagglutinable and one polyagglutinable, isolated from cystic fibrosis patients were grown in mixed culture. The strains were then separated by means of their azlocillin susceptibility and a number of colonies were tested for serotype and phage type. Changes from monoagglutinable to polyagglutinable reaction andvice versawere observed. The monoagglutinable strain was able to perform bacteriophage infection of the polyagglutinable strain, but no free phages were released. If the donor strain was to produce plaques on the receptor strain, the presence of bacteria belonging to the donor strain was essential.