Summary.115 children from an ESN school were pre‐tested on Piaget‐type conservation tasks involving seven different attributes: number, substance, length, distance, area, weight and volume. 51 of these children, who failed to conserve in relation to two or more attributes, were studied further. Aged 10 to 16 years, IQ 50–77, they were divided into three groups matched for age, IQ and level of initial understanding of conservation. One group was instructed on conservation of a variety of attributes, a second group on conservation of area only; and the third group was given practice in reading. Post‐tested after one week and then two months, 30 of the 34 instructed children consistently recognised, generalised, and gave reasons for conservation on both post‐tests. No control child improved in understanding of conservation by the time of the second