From each of two flocks (A and B) of poults comprising 14,100 females and 11,300 males, respectively, 15 poults were examined pathologically. Poults of flock A. had signs of neurological disturbances whereas birds from flock B showed respiratory symptoms. Gross lesions were observed only in two poults from flock A in which minute circular areas of cerebral malacia were seen. Histopathologically, the brain lesions contained fungal elements, and so did some of the pulmonary granulomas detected in three and six poults out of four and six birds examined from flock A and B, respectively. Mycological cultivation was attempted from the brains and lungs of five poults from flock A. However, only from the brain of a single bird a fungus, identified as Aspergillus fumigatus, was grown. Immunohistochemistry was applied because the histomorphology of fungal elements within some lesions did not offer any characteristics allowing an assessment of the identity of the infective fungi. Moreover, as fungi could not be detected within all lesions, immunohistochemistry accomplished the screening of tissues. For immunostaining of tissues a panel of monoclonal and polyclonal antibodies identifying agents of aspergillosis, candidosis, fusariosis, scedosporiosis, and zygomycosis, was used. Due to a strong and uniform reactivity of all fungal elements with immunoreagents to Aspergillus spp. an unequivocal diagnosis of aspergillosis was established in all mycotic lesions. Apart from the establishment of an aetiological diagnosis, the application of immunohistochemistry also disclosed fungal fragments in granulomas which could not be identified with conventional histochemical stains.