We have studied the acquisition and maintenance capacity of an external memory strategy in pre-school children. First, we differentiated the zone of proximal development more suited to teach the strategy. Then Normal children were compared with Down's syndrome and non-Down's syndrome retarded children. Results showed a) the existence of different levels of production deficit, which differ for the amount of aid the child requires; b) the predictive value of awareness on strategy acquisition and maintenance; c) the particular efficacy of the predominantly non-verbal teaching method of an external strategy with Down's syndrome children rather than with non-Down's syndrome retarded children.