SYNOPSISThe ultrastructure of most of the pineal eye cell types is surveyed, excepting the cell clusters (lens‐bodies) occurring in the lens and sometimes in the lumen. Greatly improved technique and apparatus resulted in interesting and new phase‐contrast observations, especially as regards the occurrence of ganglion cells in the nerve‐fibre layer and on the appearance of the sensory cell appendages. It is found that there are two types of sensory cells in the pineal eye; most of them are cone‐like cells very similar to the lateral eye cone cells. The pineal eye sensory cells have inner and outer segments, the former often having dense particles, mitochondria and oil droplets, and the latter, rod disks. The two segments are connected by a connecting cilium with ciliary filaments. The pineal eye sensory cells are obviously ciliary derivatives, and the possibility is discussed that they may develop from cilia seen to protrude through the microvillous border of the organ's lumen. It is further suggested that the outer segments may be transitory structures, or changeable in appearance.Structures resembling the two types of synapses described for the lateral eye are reported. The ultrastructure of the nerve‐fibre layer of the pineal eye is described.The inclusions and structure of the sensory cells and of pigment cells at various levels in the organ are described, and the nature of a coagulate which may represent a secretion or fluid occurring in the lumen is discussed. The fine structure of the pineal eye lens cells is described, and the absence of an inner limiting membrane noted.The possible functional significance of the above observations is