The structure of the particle of a new bacteriophage of Pseudomonas, .PHI.KZ, was studied. The particle of .PHI.KZ bacteriophage had some features distinguishing this phage from all phages known heretofore: a large size of the head and DNA and the presence in the head of a special structure termed the central body. The central body has a cylindrical shape and may be isolated from the head of the phage by repeated freezing-thawing of the phage preparation. The structure of the central body is complicated. Possible functions of the central body are discussed.