Observations have been made on the infestation by aphids of winter lettuce at Imperial College Field Station, Slough, for the seasons 1947–8 and 1949–54. The effects of transplanting and of the use of insecticides have been studied.Plants drilled in September where they were to mature were ready for cutting a little earlier than those transplanted in November, but in years when aphids were numerous the drilled lettuces were heavily infested and very few of them hearted.The infestation of young crops varied from year to year; in some winters it fell to a low level, in others it increased rapidly and persisted with serious effect on the crop. In some years lettuce mosaic virus was also prevalent. The dominant aphid species wasNusonovia ribis‐nigri(Mosley).An early and persistent attack, even in the absence of virus disease, much reduced the number of hearted plants and their average weight. A late attack by winged aphids in early spring sometimes spoilt fully hearted lettuces.Dipping the plants in nicotine solution at the time of transplanting in November reduced aphid infestation; dusting with benzene hexachloride proved more effective.If transplanting was delayed until February or March a relatively clean crop could be produced by insecticide treatment, but losses from fungal disease were usually