Gonosomal chromatin bodies (GCBs), i.e. blocks of condensed chromatin consisting of heterochromatized region of the sex chromosomes of the field vole M. rossiaemeridionalis, were used as a natural interphase chromosome marker in order to clarify the regularities of GCB rearrangement during nuclear fragmentation of secondary giant trophoblast cells (SGTCs) at the end of their differentiation. Cytophotometrical measurements of DNA content in the nuclei, nuclear fragments and simultaneously in the GCBs were made in the secondary giant SGTCs of field vole M. rossiaemeridionalis. In most cases 1 to 2 GCBs get into the nuclear fragments at different ploidy levels. In the nuclear fragments, GCB DNA content decreased mostly proportionally to DNA content in the whole fragments corresponding to 2c, 4c and 8c. The data obtained demonstrate a regular whole-genome chromosome distribution into nuclear fragments. A possible mechanism of nuclear fragmentation that largely ensures a balanced genome in nuclear fragments is discussed.