Summary.1Contrasted differences in the intensity and nature of the infestation of aphides attacking the potato crop in certain districts in North Wales prompted a detailed study of the factors involved.2Migrating alatae are considerably more numerous in the Flintshire districts than in the south Caernarvon area. The mean monthly values for relative humidity are consistently and appreciably lower in the former district than in the latter.3Controlled laboratory experiments on the effect of variation in relative humidity upon the flight ofM. persicaeclearly demonstrate an inhibitive action of high humidities upon flight. The experiments were conducted at 0, 25, 50. 70, 85 and 100 per cent, relative humidities at the temperatures 55d̀ F., 70d̀ F., 80d̀ F. and 90°F.4The findings of the survey of aphis population at centres in North Wales during 1928–33 are discussed in the light of these re