The results of a survey of aphis populations on potatoes in Devon and Cornwall; with special reference to Myzus persicae Sulz., during the years 1937-41 are given. One or more counts were made by the standard method at 107 centres to determine population index figures. The results are summarized in the form of a map. The highest index figure obtained during the 5 years was 609, and only on six other occasions was the figure of 100 exceeded; of these centres only one was situated in the area considered to be especially favourable to seed production, and this centre was a market garden. The distribution of the various species of potato aphides is discussed, together with their fluctuations from year to year; special reference is made to M. persicae and M. ornatus. Observations on the parasitism of M. persicae are described. The relationship of the proximity of winter hosts, altitude and of meteorological conditions, to the populations of M. persicae is discussed. The conditions for the production of seed potatoes in Devon and Cornwall are summarized. An account is given of those areas in Devon and Cornwall deemed suitable for seed production. A short account of the formation of a ‘Seed Potato Growers' Association’ in Devon and Cornwall has been added.