During studies of bacteriophages of Pseudomonas aeruginosa a bacteriophage with a rodshaped head was discovered. This paper will report the morphological and some biological properties of this phage. MATERIALS AND METHODS P. aeruginosa (Kitasato strain) was used as host. Forty-three strains of P. aeruginosa from the culture collection of the Department of Bacteriology, School of Medicine, Kyushu University were used for determination of the lytic spectrum of the phage. A phage of P. aeruginosa with rod-shaped head was designated as K. The host cells were cultivated in nutrient broth with aeration for 3 hr. At this time the phage suspension was added to the culture and incubation with aeration was continued for an additional 3 hr. The lysate contained approximately 108 to 1010 phage particles per ml. Plaque counting and one step growth