Summary.Potato viruses obtained from nine foreign countries have been compared with typical American forms. The chief emphasis has been placed on the potato “mottle,”“ring‐spot,” and “veinbanding” viruses.The mottle and ring‐spot viruses were found in potatoes from all nine foreign countries, but only in about one‐half of the 75 varieties or lots tested. Hence these viruses appear to be less widespread in foreign than in standard American potato varieties. The veinbanding virus was found in potatoes from six of the nine foreign countries, and was often free from associated viruses.The remarkable constancy of the viruses secured from widely different sources, when compared under identical conditions, was outstanding. Some degree of variation was found in certain cases, as for example between the veinbanding virus and theYvirus from England, though these viruses are very similar in most respects.The possible relationships of certain other potato viruses are discussed, and a potato streak virus is briefly described.Incomplete or unsatisfactory descriptions and synonomy in nomenclature are believed to be largely responsible for the existing confusion in potato virus literature. These difficulties may largely be overcome by devoting more attention to the description of the v