Neosporosis was diagnosed in an 11-year-old Quarter Horse gelding with clinical signs and diagnostic test results compatible with equine protozoal myeloencephalitis (EPM). Presumptive postmortem diagnosis of EPM attributable to Sarcocystis neurona infection is generally made on the basis of detecting an antibody titer to S neurona in the CSF or characteristic histologic lesions, even when parasites have not been specifically identified. Neosporosis was confirmed in the horse described here by use of immunohistochemical examination, in vitro culturing, and ultrastructural and molecular characterization of parasites from infected tissues. Antibody testing of serum and CSF samples indicated that Neospora-specific anti-bodies can react with S neurona proteins on western blot analysis. The confirmation that neosporosis in horses can mimic EPM emphasizes the need to broaden the etiologic definition of EPM beyond infections exclusively attributable to S neurona.