This book is written on the basis of real practical research experience gained by conducting experiment in both laboratory and field levels. Most of the examined fishes of Ailee beel were observed normal and healthy from external observations, but pathologically a great number of fishes were affected by various diseases especially EUS. The study provides very useful information regarding the clinical signs in freshwater eels, relation between disease outbreaks and water quality factors and histopathological changes of fish in different months. These will help to know the causes of disease of freshwater eels, which may be one of the factors for the fishes to be endangered day by day in Bangladesh. Therefore, it is important to develop proper preventive and control measures in order to overcome these disease problems. Steps should be taken to protect these fishes and further works are needed in this direction in order to solve the mentioned problem.