Giant bacteriophages φKZ and EL of Pseudomonas aeruginosa contain 62 and 64 structural proteins, respectively, identified by ESI-MS/MS on total virion particle proteins. These identifications verify gene predictions and delineate the genomic regions dedicated to phage assembly and capsid formation (30 proteins were identified from a tailless φKZ mutant). These data form the basis for future structural studies and provide insights into the relatedness of these large phages. The φKZ structural proteome strongly correlates to that of Pseudomonas chlororaphis bacteriophage 201φ2-1. Phage EL is more distantly related, shown by its 26 non-conserved structural proteins and the presence of genomic inversions. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.