The haemagglutinating encephalomyelitis virus (HEV) of pigs was isolated in Canada by Greig et al. (1962). Three serologically identical strains of the virus (HEV 1, 2 and 21) were detected by these workers (Greig & Girard, 1969). Cartwright (1969) suggested that HEV was a myxovirus, but Phillip, Cartwright & Scott (1971) have recently examined the virus using negative contrast methods and concluded that it belonged to the coronavirus group. This present study examines the structure of HEV as seen in thin sections, but some results obtained by negative staining are also recorded.