Viruses that infect ependyma cause ependymitis in humans and hydrocephalus in experimental animals. We report that reovirus type 1 (which induces hydrocephalus in mice) binds to the surface of isolated human and murine ciliated ependymal cells. With the use of recombinant viral clones, the binding property was mapped to the type 1 viral hemagglutinin, which also determines in vivo the affinity of reovirus type 1 for ependyma. Mumps virus, measles virus, parainfluenza type 3, and herpes simplex virus type 1 bind to murine ependyma cells, whereas reovirus type 3, herpes simplex virus type 2, and poliovirus type 2 do not.