Losses of cultured marine and freshwater fishes due to diseases averaged about 20,000 tons each year or 6% of the aquaculture production in Japan in 1980-1991. During this last decade, bacterial diseases have been responsible for most of the losses. Three trends are evident from epidemiological data. First, diseases caused by bacteria with multiple drug resistance are prevalent, and these are difficult to overcome by chemotherapy. Second, parasitic diseases and viral diseases that are practically impossible to cure are increasing. Third, some diseases seem to originate in juveniles (seed) imported from other countries. Further research should focus on: (1) improving dietary and environmental conditions, (2) giving the host animals resistance against disease through methods such as vaccination, and (3) developing diagnostic and disinfection procedures for epidemics. Active exchange of information is necessary to prevent, or alleviate the effects of, the spread of diseases through international export and import of juveniles.