The behaviour of B-chromosomes in meiosis was studied in 44 males of yellow- necked mouse, Apodemus flavicollis. The Bs behave in a non-Mendelian fashion during meiosis I i.e., appear as univalents, regardless of the number of Bs, and segregate in a random fashion. The meiotic drag of Bs, resulting from Bs lagging and premature division of Bs into chromatids, was found in 1B animals. The Bs drag amounts to about 6% of cells and is not substantial; however due to dispensable nature of Bs, it has to be compensated through drive in females or heterotic effects of Bs. This result gives support for the heterotic model of Bs evolution in A. flavicollis. On the other hand, the number of chiasmata on A-chromosomes showed increasing tendency with the number of Bs in the karyotype. As the chiasma effect is characteristic for parasitic Bs, it gives support for the parasitic model of Bs evolution. With contrasting results, the hypothesis of a combined model of Bs evolution, in which the fitness of B-bearers changes around the year or in different environmental conditions is discussed.