All viruses in the family Potyviridae which have been studied cytologically (currently 111) induce cylindrical inclusions in host cytoplasm. These inclusions are controlled by portions of the virus genome, therefore, viruses which induce them are related. Viruses in other groups do not induce this type of inclusion. Cylindrical inclusions have come to be recognized as one of the main characteristics of the family Potyviridae. They are used in diagnosis of diseases induced by these viruses. For diagnostic purposes the family can be separated into four subdivisions on the basis of differences in cylindrical inclusion morphologies. Assigning viruses to subdivisions assists in virus identification at the specific and in some instances at the strain level.