SOX9 is a member of the SOXE group in the SOX family, which plays an important role in sex determination and differentiation of vertebrates. is specifically expressed in the testes of mammals, birds, and reptiles, and is expressed in both the testes and ovaries in amphibians and fishes. To measure the expression of in the gonads of invertebrates, we cloned a 2835 bp full-length cDNA of the scallop ) which contains a 1413 bp open reading frame (ORF) encoding a 470-amino acid sequence with the HMG-box of the SOX family and the highly conserved region of the SOXE group, although there is no Pro-Gln-Ser-rich region at the C-terminus of vertebrate SOXE proteins. -SOX9 polyclonal antibodies were produced based on the full-length cDNA sequence. ISH and immunohistochemistry detection showed that SOX9 proteins were located in all germ cells of testes and ovaries at different developmental stages. The expression patterns between mRNA and SOX9 protein were similar. In the testes, the intensity of positive signals was highest in the spermatocytes and lowest in the spermatozoa. In the ovaries, the intensity of positive signals gradually decreased across the oogonia, oocytes, and mature oocytes. The expression characteristics of gonads were different from that most of vertebrates, which show variability in expression between sex, suggesting its function of gonad development and gametogenesis might be different from vertebrates.