Summary.Three questions important in relation to the developmental‐difference controversy were addressed. The first, to what extent do retarded and non‐retarded children show a similar sequence in their development of concepts, was answered using scalogram analyses which supported a common order of acquisition of conservation concepts when intermediate as well as conservation and non‐conservation categories were used. There was a high degree of similarity in sequence within as well as between attributes.The second question, how far does the similar‐structure hypothesis hold, was answered in the terms set by the developmental theorists. Multiple regression analyses and comparisons of mean mental ages for each of the conservation subgroups supported the similar‐structure hypothesis. Non‐retarded and retarded children with no known organic condition revealed similar structures.The third question concerned findings of volume conservation in retarded children and how these might be interpreted. It is argued that recognition of conservation in displacement volume tasks need not imply a generalised stage of formal operations nor be interpreted in terms of ceiling in development. Overall the findings have implications for ordering and providing appropriate levels of experiences for retarded children without assuming fixed limits in their