We studied apoptosis induced by duck enteritis virus (DEV) in vivo, focusing on the lymphoid organs that constitute the main targets for infection: thymus, bursa of Fabricius (BF), and spleen. Fifty Pekin ducks were inoculated subcutaneously with a virulent strain of DEV. The morphology of lymphoid organs of these infected ducks was observed by light microscopy and transmission electron microscopy. Cell death by classical necrosis was observed in lymphocytes of the DEV-infected thymus, BF, and spleen. Lymphocyte apoptosis also was observed at the same time, and it was further confirmed by in situ terminal deoxynucleotidyl transferase dUTP nick-end labeling and agarose gel electrophoresis. We conclude that apoptosis and necrosis of lymphocytes induced by DEV infection resulted in the depletion of lymphocytes and that apoptosis of lymphocytes may play an important role in the pathogenesis of duck viral enteritis.