The big eye kilka, Clupeonella grimmi, is a marine fish living in depth of 20 to 200 meter of the Caspian Sea. Its eye and retina were processed for histological and SEM studies. Paraffin embedded retina was cut radially and tangentially in 5 ?m thickness and stained with hematoxylin and eosin method. The unstained sections were manipulated for SEM image observations. Tangential retinal sections showed irregular arrangement of cones. Five morphologically different types of photoreceptors were distinguished as rods, short single cones, long single cones, twin cones and double cones. The cones were counted in each quadrant of the retina and cone density was determined. The cones showed increment in diameter but reduction in number at anti-clockwise direction without any specific arrangement. Presence of four types of cone cells and their pattern of distribution revealed assistance in near surface color vision and more light capture in dim light of deeper waters as an adaptation to planktivorous feeding habit and deep water living habitat.