A study was made of microcomputer application for diagnostic program of com-mon diseases in freshwawter fishes. It is according to the experiences of the fish disease expertswhich were from their practices since many years, and the computer feature of logical judgment isvery accurate and rapid. While the common fish disecses occuring the diagnostic program gives usan opportunity to diagnose them quickly without the expert's help, so that people can take emer-gency measures as quick as possible and avoid heavy losses. There are thirty-nine common diseases in freshwater fishes would be auto-diagnosing by mi-crocomputer. This diagnosing process is very convenient, it only needs to select the correct answerwhich displays on the screen while the diagnostic program is running. It takes about few minutes.The diagnostic program will give us the points for diagnosing further while the occuring fish dis-eases over the thirty-nine types in it. The hardware environments of the diagnostic program are IBM PC / XT/ AT and 100%compatibles with color CRT. The software environments of it are CCDOS and its BASICAinterpretative program.