Summary.Piaget's observations on sensori‐motor intelligence and the concept of permanent objects in the development of the normal infant were applied to the study of 147 mentally defective children. The postulated order of difficulty of responses typical of each of Piaget's sensori‐motor stages was confirmed. Repetitive behaviour with objects was readily classifiable into the types of manipulation which Piaget terms secondary, derived secondary and tertiary circular reactions; most of the hand mannerisms resembled the hand movements of infants to which Piaget has given the name of primary circular reactions. The postulated correspondence between the type of problem‐solving or grasping behaviour and the type of circular reaction was found in 43 per cent of the cases. The postulated correspondence between the stage of sensori motor intelligence and object concept development was found in 87 per cent of the cases. It was concluded that the sensori‐motor activities of severe mental defectives could be classified into the six main types distinguished by