The fine structure of the cells in the posterior segment of the vitreous body of three Hartley strain guinea pigs was investigated by electron microscopy. A total of fifty-four cells were observed. Fifty-three of the 54 cells had specific granules which manifested homogeneous or heterogeneous appearance and were considered as hyalocytes. A difference in cell activity was demonstrated among these cells. Only one cell of the 53 suggested phagocytosis. Another cell was discharging lipopigment. This suggests that hyalocyte granules are gradually degraded, becoming of more complicated structure, and are discharged into extracellular space. One of the 54 cells had cytoplasmic filaments and was not considered as a hyalocyte.