The present study demonstrated Leucocytozoon simondi as the most common blood parasites causing large scale mortalities in duckling in special area in El-Fayoum Governorate, Egypt. Adult ducks in the farm carry the parasite in chronic form without clear symptoms associated with low number of infected cells in blood. The parasite cause alteration in some physiological parameters of the infected ducks include low RBCs count, PCV, Hb, increase in lymphocytes, eosinophil and monocytes, decrease in plasma Albumin accompanied by high globulin and urea. This was associated with elevation in AST & ALT enzymes level. The study recommend regular estimation of these physiological parameters in mother ducks periodically as mean of risk assessment help in elimination of this disease from the farms via exclusions of these chronic infected ducks which serve as permanent source for this new infection in the farm.