Two genes of the oxytocin/vasopressin superfamily in the cephalopod Octopus vulgaris, cephalotocin (CT) and octopressin (OP), lack introns and consist of a single exon in their protein-coding regions, which is unlike the 2 intron-3 exon structures in most vertebrates and Lys-conopressin in a pond snail Lymnaea stagnalis. Octopus may have lost introns during the evolutionary process in mollusks. mRNA that had deleted 202 bp from preproCT cDNA (CT-del) was also expressed in the brain. The deletion occurred in the central part of neurophysin. Immunohistochemical studies suggest that a translational product of CT-del mRNA may not be present in a stable form due to the loss of neurophysin. Genomic Southern blot analysis revealed that a single copy of each of OP-, CT-, and CT-del-genes was present in the genome.