A description of structure and some bilogical properties of a new virulent Pseudomonas phage, phiKZ, is given in the paper. Phage phiKZ has unique characteristics of particle (a large size of the head and the presence in the head of a structure which is visible on electron micrographs as crosswise striated substance). A cause of high-titer phage phiKZ formation on plates has been investigated. Characteristics of single step growth experiments, sensitivity of free phage particles to heating, SDS-treatment, UV-light, cycles of freezing-thawing, osmotic shocking are determined. No exclusion is found of homologous phage at early stages of infection bacteria with phiKZ. Mutants of phiKZ with changed plaque morphology are isolated; the possibility of a recombination between the mutants is demonstrated. It is supposed that phage phiKZ may be used as excellent model for the study of genetic control of morphogenesis and packing of DNA in the phage head.