The ultrastructure and distribution of hyalocytes were examined in guinea pig eyes by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hyalocytes were distributed randomly on the vitreous surface of the retinal inner limiting membrane, where they were elongated in shape with a spherical perikaryon and a few stout processes. On the epithelial surface of the ciliary body, however, the cells were stellate with some short processes. The cells of both regions included typical dense hyalocyte granules in the cytoplasm. The surface morphology of hyalocytes indicates that the cells are wandering macrophages. The abundance of free cells in the ciliary body epithelium suggests that the area is a site for the emigration of hyalocytes or their precursors from the ciliary stroma. The homogeneous population of hyalocytes in the posterior part of the eyeball may be useful for experimental studies of the cell in vivo or their isolation for study in vitro.