A kind of bacteriophage which attacks Pseudomonas caryophylli was isolated from the affected carnation plants collected from Hyogo prefecture, Japan, and was named as CaP₁. The fundamental properties of this phage elucidated in this experiment are as follows: CaP₁ phage is tadpole-shaped, which composed of a head of ca. 56mμ in diameter and a tail of 105mμ in length and 18mμ in width. Every isolates of P. caryophylli collected from several localities of Japan were susceptible to the phage. All of the other tested bacteria, 16 species of Pseudomonas (12 phytopathogenic and 4 soil bacteria) and 11 species of other genera, were resistant to the phage. Escherichia coli was also resistant. Activity of the phage was comparatively stable under the temperatures lower than 50°C in distilled water. It was, however, rapidly inactivated at the temperatures higher than 50°C, and was completely inactivated at 57°C for 10 min. The optimum temperature for plaque formation was 30°C, under which the highest plaque forming efficiency was obtained. Nine progeny phages in average were produced from a single host cell after the latent period of ca. 90 min and the following rise period of ca. 100 min in PS medium at 30°C.