Abstract: Avian Leukosis Virus subgroup J (ALV-J) is widely described in meat-type chickens and layers type but rarely observed in ducks. In this study, two flocks of Peking duck breeder bred in Egypt showed 25-30% mortality, 20-30% drop in egg production and 60-65% drop in hatchability. Gross picture showed severe enlargement of liver, spleen, white raised nodules in heart and ovarian atrophy in all examined birds. The liver and spleen had diffuse, multifocal white raised foci on the surface as well as on the cut-surface. Histopathological examination revealed numerous myelocytes with bigger volume, large peripheral nucleus and packed reddish cytoplasmic granules infiltrated in heart, liver, kidney and ovary. Some of myelocytic cells had mitotic figures. Results were positive for detection of ALV antigen- p27 by antigen capture ELISA in cloacal samples. The PCR results confirmed that the flocks were positive for ALV-J with specific fragment of 545 bp, but negative for ALV-A, Marek’s Disease Virus (MDV) and Reticuloendotheliosis virus (REV). The study provided some information on ALV-J induced myelocytomatosis for ducks. It concluded that ALV-J virus is broadening host range including the ducks. Also, myeloid leukosis is an enduring problem facing the poultry industry.