An expandable menu driven database program has been developed to aid the differential diagnosis of diseases in fish. The program is based upon over 200 symptoms, signs, pathologic findings or characteristics of water systems. These are selected by the user from 10 menus presenting the attributes in logical groups. The program generates a differential diagnosis for any combination of entered attributes by examining profiles of over 100 diseases in the database. Diseases characterized by one or more of the attributes entered are then ranked on the basis of the percentage of entered attributes which would be expected for that disease. If a user desires further information on a specific disease, the diagnostic profile for that disease can be referenced and examined for additional signs or tests to consider in the diagnosis. A major strength of the program is its ability to expand. Menu driven help screens allow the user to alter any aspect of the database, including the addition of up to 1,100 additional diseases. The user can also alter the attribute profiles of any disease, and/or add up to 1000 additional attributes to the program. Written in compiled basic, the program requires 256 K RAM in an IBM PC/XT/AT or 100% compatible machine with two disk drives or a hard disk.