SUMMARYIn the field, caged potato plants of King Edward and Majestic cultivars infested with the potato aphidMacrosiphum euphorbiaedeveloped top‐roll symptoms, the proportion of affected plants increasing with the size and persistence of the aphid population. Yield of tubers from plots in which 90% of the plants had top‐roll symptoms was 40% less than that from control plots; yield of saleable ware was even less. Foliage produced after the aphids had been killed was symptomless even when it arose from the axil of an affected leaf. Caged field plants treated with phorate granules to prevent aphid attack did not develop top‐roll.Prolonged infestation of Pentland Crown, Majestic and King Edward plants byM. euphorbiaein a glasshouse induced rolling of upper leaves similar to top‐roll of field plants. Experimental results suggest that rolling was directly attributable to heavy attack byM. euphorbiae, not to an aphid‐transmitted