Evidence that marble spleen disease of pheasants (Phasianus colchicus) has a viral etiology was obtained from field cases using the following criteria: Intranuclear inclusion bodies in cells of the spleen, lungs, liver, and bone marrow; the presence of antigen in spleens of infected pheasants as detected by the agar gel precipitin test; demonstration of virus particles in the splenic intranuclear inclusions by the electron microscope; and the presence of specific fluorescence in spleen cells as shown by direct fluorescent antibody staining. Every case of marble spleen disease examined had all of these findings. Attempts to isolate a virus in cell culture using chicken and pheasant embryo fibroblasts and chick kidney cells were unsuccessful.