Background: Aspergillus fumigatus is considered the major agent of mycotic diseases in birds, affecting mainly the respiratory tract. It is a disease of economic importance in the poultry industry, however it is not a zoonotic or contagious disease. Aspergillus spp. are an environment residents. Infection usually occurs by inhalation of conidia released by molds, which come off the diet or specific ingredients, the nest and contamination of eggs during incubation. The objective of this study is to relate the macro and microscopic diagnosis of aspergillosis in poultry. Case: In an intensive farming of poultry (Gallus gallus), it was observed mortality rate exceeding 20%, hoarseness and difficulty breathing in males of approximately two weeks of age. The batch was treated with Terramycin ® (oxytetracyclinehydrochloride) in the first week and Trissulfin® (sulfamethoxazole, trimethoprim and bromhexine hydrochloride) in the second week. Birds were sent for analysis at the Laboratório Central de Diagnóstico de Patologias Aviárias (LCDPA) of the Universidade Federal de Santa Maria (UFSM). Necropsy was performed in three affected birds and pulmonary aspergillosis was suspected due to local pulmonary and disseminated injuries in the coelomic cavity, associated to the clinical signs. In birds assessed by necropsy examination, it was common the visualization of nodules in the internal cavity and lungs, caseous masses in the air sacs, little pigmentation on the feet and beaks and fragile bones. Portions of lung and granulomas were sent for isolation and identification of fungi in the Laboratório de Pesquisas Micológicas (LAPEMI)-UFSM and histopathological analysis in the Laboratório de Patologia Veterinária (LPV)-UFSM, where the standard protocols for each exam were used. The result of mycological examination showed Aspergillus fumigatus as the agent. The histopathological lesions observed in the lung were consistent with aspergillosis, characterized by multifocal granulomas associated with intra-lesional dichotomously branched fungal hyphae, morphologically compatible with Aspergillus sp. Discussion: The occurrence of aspergillosis depends on the dose of inhaled conidia of the fungus and the susceptibility of the host, which occurs in birds in the first weeks of age, making it more resistant to infection in adults. The history of signs consisting of respiratory distress associated with stressful situations or the recent lack of response to antibiotics may provide support to the clinical diagnosis of aspergillosis. The isolation and identification of the fungus comprises the best method to confirm the disease agent. Histopathology provides an important contribution to the morphological diagnosis of the lesion and the fungus. 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