The fine structure of retinular cells within lateral eyes ofLimulus polyphemus which had been dark or light adapted for 12 h in vivo was studied via electron microscopy. The ommatidium to ommatidium and retinular cell to retinular cell variability observed in light microscope studies was confirmed. The rhabdomeric microvilli were longer and narrower, the area of contiguous microvillar membranes greater, the endoplasmic reticulum less abundant and the mitochondrial granules (? calcium containing) more numerous in well dark adapted than in well light adapted retinular cells (Figs. 1, 3, 4, 7, 8) and membrane whorls or “vacuoles” were present in the peripheral cytoplasm of very well light adapted retinular cells (Fig. 6). Phagocytotic vesicles, multivesicular bodies and lysosomes were present in the interrhabdomeral cytoplasm of partially light adapted retinular cells (Figs. 1, 2, 3, 10). The number of retinular cell microvilli in contact with the eccentric cell dendrite was smaller in very well light adapted than in well dark adapted ommatidia (Fig. 9). The possible functional significance of these light-dependent structural changes is discussed.