Certain plants are able to convert first stage larvae of the Green Peach Aphid into apterous larvae although the larger part of them would develop into alates if reared continuously on a holidic diet.This Apterousness Promoting Principle (ApP principle) has a greater effect on previously starved larvae and on those whose extremities were partly amputated. The ApP principle can be shown most clearly in young radish seedlings, as well as in other young plants if they are optimal hosts. It is reduced in old radish leaves and in plant species which are merely accepted by Myzus persicae for a longer time. The principle is also reduced by wilting, etiolation, or during and shortly following over-population of the young seedlings. By covering the cotyledons with a thin parafilm membrane or by washing them with water the ApP principle is partly eliminated. We suggest the ApP principle is largely coupled with "good" food, but an important component of it operates as a "token stimulus" perceived already in part on the plant surface. Hence the standard diet seems to offer "poor" nutrition. With further deterioration such as the omission of ascorbic acid or brief food deprivation the number of alates increases, whereas with extremely poor food (for example if the essential amino acid isoleucine is entirely omitted or if food is withheld for more than 18 h) this gives way to an increase in the formation of apterous larvae. A group effect may modify the reaction after brief omission of food: more of the larvae in groups develop into alates, single larvae more into apters.