Abstract The penaeid shrimp develops cortical rods (CRs) in the oocyte after completing yolk accumulation during ovarian maturation. In this study, CRs from the mature ovary of the kuruma prawn ( Marsupenaeus japonicus) were isolated and their proteins were characterized. Under reducing conditions of SDS-PAGE, the CR demonstrated five major bands of approximately 210, 150, 140, 130 and 30 kDa. Of 32 clones of monoclonal antibodies raised against the CR proteins, 21 were found to be immunoreactive by Western blotting, demonstrating 150, 140 and 130 kDa proteins simultaneously. This result suggested that the proteins were post-translational or post-transcriptional products from the same gene, or that they originated from the same gene family. CRs were identified by immunohistochemical analysis by 31 clones of monoclonal antibodies in paraffin sections and by 26 clones in cryostatic ones. Most of the antibodies also stained the yolk in both developing and mature oocytes, suggesting that CR proteins accumulated in developing oocytes, then assembled to form the CR structure.