Abstract: Using streptavidinbiotin complex immunoperoxidase staining, the tissue localization and distribution of a highly pathogenic avian influenza virus, A/duck/Guangdong/220/2004(H5N1) were investigated in ducks inoculated by intravenous and ocularnosaloralcloacal routes. The results showed that the viral nucleoprotein antigen was most often detected in the cardiac muscle cells, hepatocytes, Kupffer cells, pancreatic acinar epithelial cells, renal tubular epithelial cells, vascular endothelial cells and phagocytic leucocytes, and occasionally in the mucosa epithelium of trachea, bronchia and intestinal tract, encephalon, and macrophages, histiocytes and necrotic cellular debris of thymus, bursa of Fabricius, spleen and lung. But no viral nucleoprotein antigen was detected in the oesophagus, stomach, testis, ovary, thyroid glands, adrenal glands, Harderian glands and skeletal muscles.