From January 1999 to September 2000, a total of 494 cases of eel diseases were diagnosed in central Taiwan. Among them, 437 cases belonged to A. Anguilla and 57 cases were A. rostrata and A. Anguilla. All the cases were categorized to: (1) bacterial infections: 41.5% (Aeromonas spp., Edwardsiella tarda, Flexibacter spp., Vibrio spp., Streptococcus spp.); (2)parasitic infestations: 33.4% (Anguillicola spp., Cyrodactylogyrus spp., Pseudodactylogyrus spp., Trichodina spp., Pleistophora spp., and others); (3)mycotic (Branchiomyces spp. and Saprolegnia spp.); (4)poor water quality: 13.4 %, and (5)others 6.3%. Most of the infectious cases occurred in summer, May to September, although most Many of the pathogens were noticed as opportunistic bacteria and parasites. All the cases submitted for disease diagnosis were conducted by routine procedures including pathological and microbiological examinations, and water quality analysis.