Bacterial diseases are the most common diseases among cultured fish. Aeromonas hydrophila considered the most important freshwater bacterial fish pathogen. Early diagnosis considered the cornerstone in controlling the disease. Biochemical method is laborious, expensive and time consuming whereas agglutination test is simple, rapid, accurate inexpensive and the test can be performed in field conditions. In the present study, agglutination test was established for diagnosis A. hydrophila infection in Oreochromis niloticus. A. hydrophila was isolated from naturally infected fish then distracted to subunits somatic and flagellar antigens. The prepared antigen was reacted with positive antisera of A. hydrophila which produced distinct tiny clumps. The sensitivity and specificity of slide agglutination test against collected sera were 90%, 77.5% respectively in case of somatic antigen and 92.5%, 72.5% in case of flagellar antigen. It could be concluded that agglutination test can be used as screen test for diagnosis of A.hydrophila infection in fish farms and the method can be diagramed on other bacteria.