In diploid Cochlearia pyrenaica, its established natural autotetraploid C. officinalis, and their newly induced autotetraploid and auto-octoploid derivatives, B chromosomes change the normal pattern of chromosome association by suppressing homologous pairing. Frequency of bivalents increases at the expense of multivalents from lower to higher numbers of B chromosomes. The reduction of multivalents due to the direct influence of the B chromosomes, independent of pollen mother cell chiasma frequency, is suggested as being related to the mechanism that prevents A/B chromosome pairing.