This study was undertaken to describe clinical, mycological and histopathological findings in black neck ostriches affected with severe aspergillosis in a flock including 80 birds, near Tehran, Iran. The signs included anorexia, depression, notable weight loss, diarrhoea, severe respiratory distress and death. Grossly, the lungs showed numerous white to yellow caseous nodules and the walls of the thoracic and abdominal air sacs were thickened with inflammatory exudates containing cellular debris, necrotic masses and green mold colonies. Multiple nodules were observed in the liver, spleen and gastrointestinal tract as well. Histopathologically, there were conidial heads and fungal hyphae in the air sacs and multifocal necrotic and granulomatous lesions with septated and dichotomously branched hyphae in various tissues, which were stained with haematoxylin and eosin and Grocott's methenamine silver nitrate. Aspergillus fumigatus was isolated in various tissues taken from affected ostriches.