Abstract: Among populations of Callosobruchus maculatus, there are two distinct forms of adult of each sex. One is very active in its behaviour and can fly, while the other is of rather dark body colour and cannot fly. The forms differ not only in morphology but also in physiology, chemical constitution of the body, behaviour and the response to crowding. The flight form can be induced by exposing the larval stage to crowding, high temperature, food of low water content, or to very short or very long photo-periods. A hereditary element controlling the appearance of the forms was experimentally demonstrated.