A disease of suckling piglets, which was characterised by encephalomyelitis, was discovered in Ontario in 1957. In 1958 it reached serious proportions thus predicating the need to determine its nature and cause. With these aims in view the work presented in this thesis was undertaken. The disease is here called "Ontario Encephalomyelitis of Swine" or "Ontario Encephalomyelitis" to avoid confusion with other porcine encephalomyelitides in later discussion but it is not meant to imply that this is a new disease of pigs. This thesis contains a description of "Ontario Encephalomyelitis of Swine", a comparison of this with other known procine encephalomyelitides and the details of the investigations of its cause.