SummaryAlthough virusXis carried by the potato almost without symptoms and was first discovered only twenty‐two years ago when it was already nearly ubiquitous, its history outside South America can be traced over the centuries with some assurance of accuracy. Potatoes of diverse varieties were brought to Basutoland and Tristan da Cunha in the last century and since grown in isolation. They are free from virusXwhich, because the virus is not thrown off by clones which have become infected, indicates a high general level of health at that time. The change which has come about elsewhere since then is not merely fortuitous. A hundred years ago varieties were innumerable and mostly short‐lived; and between changes of variety there was normally little chance for the virus to accumulate. Nowadays the varieties which are extensively cultivated are relatively few and mostly long‐lived. With their longevity has come the chance for the virus to accumulate; and the accumulation has been helped by a seed industry which controls leaf‐roll and severe mosaic, thus obviating the need for changing varieties, but not virusXexcept in its more severe strains. American varieties were apparently infected early: too early for the history of the attack to be traced. British varieties seem to have been attacked later, and the evidence of ‘Up‐to‐Date’, with a hint of confirmation from ‘Champion’, is that infection was scarce until about the second decade of this century.Losses from virusX, conservatively put at 800,000,000 bushels a year for the world over, are both an indictment of the organization of the seed industry and a warning against the danger of abandoning varietal abundance and novelty in crops which are propagated