The joining (J) chain is an acidic polypeptide and component of polymeric, but not monomeric immunoglobulin molecules and may play a role in the polymerization of mammalian immunoglobulins. However, the function of J chain in immunoglobulin polymerization has been questioned. In human ontogeny, the J chain has been detected in the fetal liver at the sixth gestational week, and its expression was found to precede synthesis of μ chain at least by 1 week. It has been reported that J chain mRNA could be detected in invertebrates. In this study, we examined the ontogeny of J chain expression in silkworm, Bombyx mori that has no immunoglobulin production. In northern blot analysis, the J chain gene expression was found in the first stage of larvae to adult moth. RT-PCR revealed the presence of J chain mRNA in 6 days embryo after fertilization. Immunohistochemical study revealed that the J chain was localized in intestinal epithelial cells.These results indicated that the J chain may possess an another function in addition to immunoglobulin polymerization.