Human placenta samples obtained at 4, 6, 8 and 10 weeks of gestation and at full term were studied by transmission electron microscopy (TEM). The chorionic villi at early stages of gestation exhibited abundant microvilli on the free surface of the syncytium. The chorionic villi at full term were covered by syncytium which bore short microvilli on its free surface. The syncytial cells were remarkably higher in density than Langhans' cells. In the earlier period of gestation, a dividing membrane was noticed in the inside of the syncytial cells. The cytoplasm of the syncytial cell contained comparatively small mitochondria, well-developed endoplasmic reticulum cells, ill-developed Golgi systems, electron-dense lipoid droplets and plenty of ribosomes. A great number of Langhans' cells were noticed in early gestation; only very few were noticed the later part. Mitochondria of Langhans' cells were comparatively large and the Golgi system was well-developed in the first trimester. Endoplasmic reticulum was less well developed and there were few ribosomes. The basement tended to become thicker a pregnancy proceeded and lamellar structures appeared on its inner face.