We have cloned a cDNA for an opsin (Boceropsin) from the silkworm larval brain which was suggested to contain the photoperiodic receptor. Its deduced amino acid sequence was composed of 381 amino acids and included amino acid residues highly conserved in insect visual pigments. This opsin belonged to the long wavelength photoreceptor group of insect opsins, and are presumed to be photoperiodic receptor. RT-PCR analysis revealed that Boceropsin mRNA is expressed in the larval brain, but not in the subesophageal (Sg) and thoracic ganglion. Immunohistochemical analyses demonstrated that Boceropsin protein is present bilaterally in some defined cells localized in the brain of the Bombyx larva. Boceropsin was considered not to be involved in the circadian photoreception, because carotenoids are not indispensable for the photoreception and formation of circadian rhythms in the silkworm.