The gene expression pattern of differentiated oocytes is reprogrammed into that of totipotent preimplantation embryos before and/or after fertilization. To elucidate the mechanisms of genome reprogramming, we investigated histone H3 lysine 79 dimethylation (H3K79me2) and trimethylation (H3K79me3) in oocytes and preimplantation embryos via immunocytochemistry. In somatic cells and oocytes, H3K79me2 was observed throughout the genome, whereas H3K79me3 was localized in the pericentromeric heterochromatin regions in which there are no active genes. Because H3K79me2 is considered an active gene marker, H3K79 methylation seems to have differing functions depending on the number of methyl groups added on the same residues. Both H3K79me2 and H3K79me3 decreased soon after fertilization, and the hypomethylated state was maintained at interphase (before the blastocyst stage), except for a transient increase in H3K79me2 at mitosis (M phase). H3K79me3 was not detected throughout preimplantation, even at M phase. To investigate the involvement of H3K79me2 in genome reprogramming, somatic nuclei were transplanted into enucleated oocytes. H3K79me2 in these nuclei was demethylated following parthenogenetic activation. However, the nuclei that had been transplanted into the parthenogenetic embryos 7 h after activation were not demethylated. This suggests that the elimination of H3K79 methylation after fertilization is involved in genomic reprogramming.