Sixteen phage suspensions isolated from the sputum of sixteen cystic fibrosis patients and five phages from the present phage typing set were studied by electron microscopy. All sputum samples contained at least one type of bacteriophage (range: 1-4) which could be classified by the morphology and dimensions of the virion. All phages isolated from sputum as well as the four typing phages were tailed phages. The clinical phages belonged either to the Myoviridae, Siphoviridae or Podoviridae family. The four typing phages belonged to the Myoviridae family. The detection of the presence of tailed phages in sputum further supports the prevailing theory that phages play a role in the phenotypical change of Pseudomonas aeruginosa during the chronic lung infection of cystic fibrosis patients, since only tailed phages are known to be temporate and thus mediate transduction and conversion.