The adult lamprey retina has two types of photoreceptor cells, short and long photoreceptor cells, which are equivalent to rods and cones of other vertebrates. In contrast, the retina of lamprey larvae only contains a single type of photoreceptor cell, which appears to correspond to the short photoreceptor cell. However, the developmental pattern of the long photoreceptor cell is unknown. Previously, we reported that antibodies against rhodopsin and iodopsin (the chicken red cone opsin) could discriminate between the outer segments of short and long photoreceptor cells, respectively, in the retina of adult Japanese river lamprey (Lethenteron camtschaticum). Here, we immunohistochemically investigate the appearance of long photoreceptor cells in the larval and adult retinas of the Far Eastern brook lamprey (Lethenteron reissneri), which is a close relative of the Japanese river lamprey, by using anti-iodopsin antibody. We found that iodopsin immunoreactivity was localized not only in the adult retina but also in the larval retina. Moreover, we examined the immunohistochemical localization of signal transduction molecules, such as transducin and arrestin, in the iodopsin-immunoreactive cells of the larval retina. The iodopsin-immunoreactive cells also contained both transducin and arrestin, suggesting that long photoreceptor cells are already functional in the larval stage before the acquisition of visual function. Our results suggest that the iodopsin-immunoreactive cells may be related to not only cone vision in the adult but also photoreception in the larval lamprey.