The three-dimensional structure of the Pseudomonas aeruginosa bacteriophage phiKZ head has been determined by cryo-electron microscopy and image reconstruction to 18A resolution. The head has icosahedral symmetry measuring 1455 A in diameter along 5-fold axes and a unique portal vertex to which is attached an approximately 1800 A-long contractile tail. The 65 kDa major capsid protein, gp120, is organized into a surface lattice of hexamers, with T = 27 triangulation. The shape and size of the hexamers is similar to the hexameric building blocks of the bacteriophages T4, phi29, P22, and HK97. Pentameric vertices of the capsid are occupied by complexes composed of several special vertex proteins. The double-stranded genomic DNA is packaged into a highly condensed series of layers, separated by 24 A, that follow the contour of the inner wall of the capsid.