The ovulation of diploid oocytes, abnormally arrested at or during the first meiotic division, is an inheritable trait (DiplI) in mice and modulated by a maternally transmitted factor. By repeated backcrossing, mouse strains with identical nuclear encoded genes and differing only in their mitochondrial genomes can be created. NMB mice represent such a strain having acquired the nuclear genome of C57BL/6J but still possessing mitochondria and therewith mitochondrial DNA (mtDNA) of NMRI/Bom, their female progenitor. The strains NMB and C57BL/6J were used to characterize a new mitochondrial trait, namely the ability to modulate the expression of the inheritable meiosis I error Dipl I in oocytes. We show that an increased rate of ovulated diploid oocytes is associated with the mtDNA type of C57BL/6J. These results corroborate the assumption that mitochondria do play an important role in meiosis of mammalian oocytes and hence seem to be involved also in the orderly segregation of chromosomes.