SummaryExposure of the Peruvian potato varieties to full summer‐day conditions at Cambridge resulted in a more uniform growing period, and increased the tendency to bloom. The effect on tuber production varied according to the variety. In most types the weight and number of tubers were not adversely influenced, but there was a tendency to elimination of the dormancy period and to conversion of the stolons into aerial shoots. In a few varieties there was no adverse influence, and in the members of one small group plants exposed to full‐day conditions formed no tubers.Investigation of virus content showed that of fifty‐nine varieties only eleven were healthy. In the remaining forty‐eight varieties there were found viruses apparently identical with those known in this country asX, B, C, F, Gand perhaps leaf roll.1There was also evidence of the presence of other viruses, suggesting the existence in South America of a virus complex, only isolated members of which have hitherto been known in Europe. It follows that great caution should be exercised in introducing South American varieties into potato‐growing