Endopolyploidy, which arises through the duplication of DNA without accompanying nuclear division, occurs in large numbers of lower and higher plants and animals, including the best known, the salivary gland nuclei of Drosophila. Endomitosis is one of the processes leading to endopolyploidy, in which the stages of mitosis (prophase, metaphase, anaphase) take place inside the nuclear membrane and without spindle formation. In mammals, endomitosis has been observed in the trophoblast of the placenta of the mouse, rat and rabbit. This is the first report of endomitosis in a normal human tissue, the trophoblast of first trimester human placenta.