A reo-like virus (calf rotavirus) was shown to be associated with cases of neonatal calf diarrhea in France. The virus could be detected in more than 50 p. 100 of diarrheic fecal samples, while it was practically absent in control samples originating from healthy calves that had never had diarrhea. The results obtained by electron microscopy and immunofluorescent studies indicate that the virus is closely related or identical to the agent isolated originally in the United States.