This work aims to assess the difficulties encountered by students of the Ecole Normale Superieure of Kouba (Algeria) intending to teach physical science in the integration of the hybridization of atomic orbitals. It is a concept that they should use in describing the formation of molecular orbitals ([sigma] and [pi]) in organic chemistry and gaps in the mastery of these concepts may represent an obstacle for the interpretation of the reactivity of organic compounds. Several studies have noted that the concept of hybridization is among one of the most difficult to understand for students at all levels of learning chemistry. In this work we try to analyze the alternative conceptions that students have constructed and how they have brought together in a conceptual structure the various concepts related to hybridization. The analysis of responses to a written questionnaire and exchanges between students in group activities sequences shows that for most students, the hybridization is not assimilated correctly. It seems that many students can speak about hybridization only once the bonds are formed and there is confusion between the formation of hybrid orbitals and the formation of molecular orbitals. Moreover, various alternative conceptions for the hybridization and the meaning of the designation of hybrid orbitals (sp, sp[superscript 2], sp[superscript 3]) appear. Finally, from the reasoning used by students in achieving the proposed tasks, we inferred a knowledge structure of their possible integration of the concept of hybridization. (Contains 2 figures an 8 tables.)