Summary : Symmetrical differences, about its development. A classification task was used to create two groups out of 24 children aged 6 to 8 ; twelve of them could order objects in collections and twelve of them could organize collections along some dimensions. These children answered questions about intersections, single differences and symmetrical differences in two tasks. An ontogenetical and cognitive approach was used to analyse the results. The sharp distinctions between the two groups and the hierarchy of the performance are consistent with the constructivist theory but show different levels in the concrete stage. Computerization of the results helps establishing models, systematizing observations and brings out the process of the children's evolution. Key words : ontogenesis, classification, strategy, computer science.