This study was conducted to isolate the pathogens and to screen the bacteriophage (phage) for therapeutic application. A total of 36 W. ceti strains were isolated successfully from the diseased fish and confirmed by biochemical tests, 16S rRNA and PCR assay. Herein, in the first time, the Pwr-1 phase was screened and isolated successfully, which is a novel virulent bacteriophage pathogen as a good candidate for therapeutic application. The phage belonged to the family of Siphoviridae, possessed an isometric head approximately 65nm in diameter and a flexible, non-contractile tail of 170-180 nm in length. Latent time and burst size of phage was about 25 min and 16 PFU/ infected cell, respectively. Phage exhibited to be relatively stable with wide range of pH and temperature. Phage had wide host range, lysed all 36/36 tested W. ceti strains. This is the first study to investigate a virulent bacteriophage of newly emerging pathogen infecting rainbow trout with various features making it become a highly potential agent for the therapeutic application in the future alternatives for antibiotic uses.