Of 35 nutria (Myocaster coypus) at a city zoo, 20 died or were killed because of a progressive central nervous system disease. On the basis of histopathologic findings, a diagnosis of cerebral nematodiasis complicated by nosematosis was made. The only significant management change prior to illness was the feeding of poplar tree branches from a lowland area inhabited by skunks and raccoons. Neurologic disturbances gradually ceased within 2 months after control measures were initiated and the practice of provided poplar branches as a source of roughage was discontinued.