Protozoan encephalomyelitis was diagnosed post mortem in a five-day-old Friesian calf which had shown nervous signs from birth. The lesion was a subacute necrotising multifocal encephalomyelitis associated with protozoan bodies (6 X 6 microns to 16 X 30 microns). Ultrastructurally these bodies corresponded to apicomplexan meronts composed of eight to 89 merozoites which reproduced by internal budding to form paired daughter cells (endodyogeny). The merozoites were indistinguishable from Toxoplasma gondii; they reacted weakly with anti-Sarcocystis species serum and did not cross react with anti-T gondii serum. The generic identity of the sporozoan was not established, but it is unlikely to have been either Sarcocystis species or T gondii.