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Seven potato (Solanum tuberosum Lin.) varieties previously indexed against potato leaf roll virus (PLRV), potato virus Y (PVY), potato virus S (PVS) and potato virus X (PVX) were grown in the fieid for two seasons at Namulonge representing warm, mid-altitude tropics and, for one season at Kalengyere representing cool highland areas. One set of the seven potato varieties was sprayed with dimethoate at a rate of 1.19 kg a.i. ha-1 at weekly intervals to prevent aphid infestation and spread of aphid transmitted viruses. The second set was not treated. Seed tuber indexing before planting showed the absence of aphid transmitted viruses (PLRV, PVY and PVS) in seed potato obtained from Kalengyere during both seasons. Weekly collection of aphids at both sites showcrl a predominance of Myzus persicae (Sul.), the principle vector of most potato viruses. Dimethoate controlled aphids at Kalengyere but not at Namulonge. As a result, seed potatoes obtained from Kalengyere and grown at Namulonge acquired the aphid-transmitled PLRV in the fint season of growth whether they were treated with the insecticide or not. However, all the varieties were free from PVY and PVS. At Kaleogyere, all the varieties grown in the field were free from aphid-borne viruses irrespective of the insecticide fre:ltment. These results indicate that there·is probably M. persicae resistance to dimethoate at Namulonge.