Recently, a crustacean female sex hormone (CFSH) was purified from the female eyestalk of the blue crab Callinectes sapidus. This report suggested that CFSH plays an essential role in the development of female secondary sexual characteristics. In order to broaden current knowledge of CFSH, here we report the cloning of the cDNA encoding a putative crustacean female sex hormone in Marsupenaeus japonicus (Maj-CFSH). The deduced Maj-CFSH precursor showed identical composition with other CFSHs, including a signal peptide, a CFSH-precursor-related peptide (CPRP), a dibasic processing signal and a mature Maj-CFSH. Although the amino acid sequence identities between Maj-CFSH and C. sapidus CFSH was low (38.9%), positions of eight cysteine residues and the N-glycosylation motif were found to be conserved between the two sequences. Tissue distribution showed that Maj-CFSH was expressed in both female and male eyestalks with roughly the same level. Analysis of in situ hybridization showed that Maj-CFSH was expressed in neurosecretory cells in the medulla terminalis ganglionic X-organ (MTGX), medulla interna ganglionic X-organ (MIGX), as well as neural cells around the medulla externa in both female and male eyestalks. Our results suggest that Maj-CFSH is not a female-specific molecule and, therefore, is a CFSH homolog, which may have various, as yet undetermined functions in males, as well as females of M. japonicus.