Electron-microscopical studies on the causative organism of contagious pustular dermatitis in sheep and bovine papular stomatitis, employing negative staining, shadow casting, thin sectioning and enzymatic analysis, revealed the typical ultra-structure of the pox viruses. This ultrastructure had been studied previously in the vaccinia virus, but there are distinct differences in these structures between the vaccinia virus and the viruses described here. The surrounding membrane and the filaments are thicker, and the double element, the peripheral protein layer and the inner core with the triplet of tube-like strands are smaller than those of the vaccinia virus, and thus correspond to the reduced dimensions of the virions. No differences were found with respect to the anomalous forms of the virions, the behavior toward proteolytic enzymes and the developmental stages. Shadow casting showed that the filaments in the membrane do not cross but that the corresponding images result after negative staining by superimposure of the upper and lower sides. It is proposed that the Orf and the BPS viruses, as well as the virus causing milker's nodules, be classified as a sub-group called para-vaccinia viruses.