Despite an average difference of about 50% in DNA amount, homoeologous chromosomes pair effectively at first metaphase in the diploid interspecific hybrid between Lolium temulentum and Lolium perenne. However, in the presence of accessory B chromosomes and “diploidising genes” pairing at metaphase I is severely reduced. Reconstruction of serial electron micrographs through pollen mother cell nuclei show that synaptonemal complexes are formed at pachytene between not only homoeologous but also non-homologous chromosome segments resulting in multivalent formation. These associations are largely ineffective in terms of chiasma formation and degenerate at late pachytene. It is highly probable that the pairing determinants exercise their control on chromosome pairing largely by prohibiting the siting of crossovers in homoeologously paired chromosome segments.