Fish disease diagnosis is a complicated process and requires high level of expertise. Any attempt of developing a web-based system dealing with disease diagnosis has to overcome various difficulties. This paper describes a Chinese National Funded Research Project (863 project) aiming to develop a web-based intelligent diagnosis system for fish diseases. The paper explains the need for a web-based expert system, the fish diagnosis process and the difficulties involved in developing the system. The system structure and its components, such as database, knowledge base and image base and their functions are described. The system has over 300 rules and 400 images and graphics for different types of diseases and symptoms. It can diagnose 126 types of diseases amongst nine species of primary freshwater fishes. The system has been tested and is now in pilot use by fish farmers in the North China region. Some issues on developing web-based expert systems from the experience gained from the research are discussed.