Summary : Evolution of childreh's behavior in numerical identity tasks. The role of the number of objects, of their position in space and of the type of action effectuated were studied in two experiments. Forty six and one hundred and ten children (from age 3 to 8) were examined in different situations of numerical conservation. The results show that the number of objects and their spatial disposition become less and less important between ages 3 to 7. A passage can be seen between the possibility to establish the numerical identity of two different objects through comparison and that of maintaining the numerical equivalence of two collections. The gradual increase of the number of successes testifies to a gradual transition between two levels of invariants. This transition also seems to be influenced by the nature of the task. The children maintain their judgement of numerical equality more easily when they place pearls in each glasss simultaneously than when they have to put the same number of pearls as the experimenter. Key words : invariant, numerical conservation, ontogenesis.