The usefulness of the bacteriophages as a biological control agent for the culture fish diseases is based on the following reasons: (i) The growth rate of bacteriophage is much faster than bacteria; (ii) The infection of bacteriophage occurs in aqueous state; (iii) The infection of bacteriophage is very specific in respect to its host; (iv) There is no problem of drug residues and drug toxicity as with the use of chemotherapy. We have initiated the isolation of bacteriophages to infect Aeromonas hydrophila which is the pathogen of eel's red-fin disease. Among the eight isolated bacteriophages, AH1 has strongest bacteria-lysis ability. Therefore, AH1 was selected as the experimental model system for the study of biological control of diseases. The one-step growth curve showed that AH1 started to form phage particles after 50 min of infection and completed at 100 min with a burst size of 160. It means that one AH1-infected A. hydrophila can produce 160 phage particles. The A. hydrophila concentration was reduced 2×10⁶ times compared with the starting concentration after infection with phage AH1 at an M.O.I. =1.2. In order to test the loss of pathogenecity of A. hydrophila after AH1 infection, the AHl-infected bacteria were injected to loach Misgurnus anguillicaudatus. After 3 hr infection of AH1, the A. hydrophila had completely lost its infectivity and mortality in the injected loaches. Even the multiplicity of infection (ratio of bacteriophages to bacteria) lowered to 0.001, the infectivity and mortality were reduced to 40 % of uninfected A. hydrophila. The bacteriophage AH1 viral particles were very stable in sterilized tap water, less stable in fish pond water and most unstable in distilled water.