The J chain is an acidic 15 kDa protein and its function is considered to be involved in the polymerization of immunoglobulin (Ig). Since it has been reported that development of J chain ontogenically precedes the synthesis of μ chain and that the production of IgM occurs without J chain, J chain may have another function in addition to Ig polymerization. The amino acid and DNA sequence analysis indicated that J chain has a high homology among various animal species. In this study, we have examined the expression of J chain in earthworms which do not produce Ig. Our results indicated that J chain is present in the earthworm with a high homology compared to DNA sequences of other higher animals. Immunohistochemical staining revealed that J chain is localized in the epithelial cells of skin and intestine. From these results, the function of J chain in the earthworm may not be the polymerization of Ig.