In Southeast and East Asia, Leucocytozoon caulleryi, a protozoan of the order Haemosporida of the phylum Apicomplexa, causes anemia, hemorrhage, and mortality in young chickens, and anemia and hemorrhage with egg drop and soft-shelled eggs in layer hens. Leucocytozoonosis caused by L. caulleryi is a notifiable infectious disease in Japan because of the damage it causes to the poultry industry. During the summer, biting midges (Culicoides arakawae) transmit this protozoan to chickens. This review provides the etiology, pathology, and diagnosis of L. caulleryi infection in chickens.