[en] The significance of programmes concerning species resistance are discussed on hand of the relations between lucerne (Medicago sativa L.) and aphids. In the USA this method is utilized to combat the losses resulting from the accidental introduction of the plant louse. In Europe, intensified forage production justifies its use since it avoids pollution and reduces the cost of plant protection for the producer. The various stages of the programme are analyzed. The case of the pea aphids (Acyrthosiphon pisum Harris) is discussed specifically on hand of published literature and our own results. The first point must be an evaluation of the destruction caused by the pest. Subsequently adequate methods for mass rearing of the insect must be available. The conditioning of bean (Vicia faba L.) as host instead of A. pisum should be treated with reserve. Furthermore, trustworthy and reproducible tests are required. As far as we are concerned, we evaluate the net rate of reproduction of the aphids. The selection of lucerne strains of sufficiently high genetic resistance should make it possible to avoid problems arising from variability in aphid populations. Various examples are given of such modified resistance resulting from the selection of new biotypes. At present one can only make assumptions as to the causes of strain resistance to aphids in lucerne. In the case of M. sativa, selection programmes appear favoured by the great variability of the species