Allopolyploids constitute a large proportion of flowering plant species. They are the product of natural or synthetic hybridization between related diploid species or genera, and therefore contain three or more sets of chromosomes which are structurally and genetically different. Many allopolyploids are highly fertile and show disomic inheritance, because chiasma formation at first metaphase of meiosis is restricted to homologous chromosomes only. Observations of synaptonemal complexes during meiotic prophase have shown that the diploidization of these species can be achieved in several ways depending upon the synaptic behaviour of chromosomes and the strict localization of chiasmata. These strategies of bivalent formation are described and compared in several allopolyploid species, with particular emphasis upon the relative importance of structural and genotypic components of the process.