A captive fallow deer (Dama dama) in a zoo was spontaneously poisoned after consumption of Sida carpinifolia. The paddock where cervids were kept was severely infested by S. carpinifolia. The deer developed a neurological syndrome characterized by muscular weakness, intention tremors, visual and standing-up deficits, falls, and abnormal behavior and posture. Because a severe mandibular fracture and the consequent deteriorating condition, it was euthanized. Main microscopic findings were swelling and multifocal cytoplasmic vacuolation in the Purkinje cells. The cytoplasm of multiple cells of the cerebellum, especially the Purkinje cells, stained with the lectins Concanavalia ensiformis, Triticum vulgaris, and succinylated Triticum vulgaris. Diagnostic possibilities such as bovine diarrhea virus, rabies, and transmissible spongiform encephalopathy were excluded. The report focuses on the risk of maintaining S. carpinifolia populations in zoo enclosures of wild herbivores.