The restoration of the distribution range of the gray wolf (Canis lupus) creates the possibility of gene exchange in the wolf’s various populations, and biodiversity in the environment. At the beginning of May 2020, in the Turek Forest District, an atypically behaving young individual was registered. The wolf was found dead after four days of observation. A radiographic examination of the body was performed, followed by an autopsy and histopathological and cytopathological examination of the lungs and pleural effusion. Post-mortem examination revealed a significant amount of cloudy fluid with blood in the pleural cavity. The lungs were sunken and showed a series of lesions indicative of inflammation. The diagnosis was confirmed by a histopathological examination and smear of pleural fluid. The laboratory tests revealed the presence of bacteria with the morphological features of Streptococcus spp. and Enterococcus spp. The changes found in the study indicate that fibrinhaemorrhagic- necrotizing pleurisy was the primary process and the cause of the wolf’s death. The circumstances described indicate that the tested wolf was an individual rejected by the pack, who migrated alone. Decreased immunity, possibly caused by stress and malnutrition, caused the proliferation of opportunistic bacteria that eventually led to pleurisy and death.