Abstract: The relationship of extracellular Ca concentration and the fine structure of dark adapted photoreceptors in the crab (Eriocheir sinensis) was studied with electron microscopy.The results showed that in vitro,in presence of 50 mmol/L calcium,the diameter of rhabdoms and the dimension of perirhabdomal vacuoles extremely reduced than that of photoreceptor incubated in physiological saline.The dimension of the cytoplasm,located around the rhabdom,increased in the higher extracellular Ca solution.Meanwhile,like the morphological structure of the retinular cells during light adaptation,the pinocytotic vesicles were more,the diameter and number of multivesicular bodies reduced,while the number of lamellar bodies and lysosomes increased in the retinula.The pigment granules were more and located in distal part of the retinular cell.However,in presence of 50 mmol/L EGTA and 2 mmol/L calcium,the diameter of the rhabdoms,the dimensiom of perirhabdomal vacuoles and the number of multivesicular bodies was increased,but the pinocytotic vesicles were few and the pigment granules located in the proximal region.