Inclusions characteristic of an infection caused by Toddia sp. were found in the erythrocytes and erythroblasts of 15 of 26 northern water snakes (Nerodia sipedon sipedon) collected near Kingston, Ontario, Canada. Erythrocytes contained translucent inclusions, small acidophilic bodies, and square-shaped crystalloid structures. Erythrocytes infected with Toddia sp. were more rounded than uninfected erythrocytes and had pycnotic nuclei. We observed icosahedral virus particles measuring 195 to 210 nm formed from a membrane-bounded viral assembly site in the cytoplasm of the host erythrocyte. As a result of the viral identity of this parasite, we recommend that the etiologic agent of Toddia sp. infections from this and other species of North American snakes be renamed Snake Erythrocytic Virus.