Egg-laying hormone (ELH) is a neuropeptide secreted from the neural ganglia of mollusks that can stimulate the egg-laying and associated behaviors. In the present study, using a polyclonal antibody against an abalone, H asinina, ELH (aELH) we could detect the aELH-like immunoreactivity (aELH-l-ir) in the central nervous system (CNS) and ovary of the blue swimming crab Portunus pelagicus. In protocerebrum, aELH-l-ir was detected in the small- and medium- sized neurons of cluster 6, anterior/posterior medial protocerebral neuropils, protocerebral bridge and central body. In deutocerebrum, the aELH-l-ir was observed in small-sized neurons of cluster 9, fibers between small-sized neurons of cluster 10, medium-sized neurons of cluster 11, olfactory neuropil and medial/lateral antenna I neuropils. In tritocerebrum, the medium- sized neurons of cluster 16, the tritocerebral commissure, and tegumental neuropil showed strong aELH-l-ir. In the ventral nerve cord, the aELH-l-ir was detected in the small- and medium-sized neurons of dorsolateral clusters of subesophageal ganglion. There was no aELH-l- ir in neurons of thoracic ganglion, but aELH-l-ir exhibited a dot-like structure in the subcapsular space around the thoracic neuropils. In abdominal ganglion, the aELH-l-ir was detected in some medium-sized neurons and abdominal neuropils. In the ovary, the aELH-l-ir was detected in the cytoplasm of oogonia, and all steps of developing oocytes. The effect of aELH on ovarian maturation in the crabs was examined by bioassay. When mature female crabs were injected with a dose of 500 ng/g body weight per crab, the ovarian maturation was accelerated as indicated by the rising of gonado-somatic index and levels of vitellin in the ovarian tissue and hemolymph, which was confirmed by histological studies. The aELH also enhanced the proliferation of follicular cells as indicated by the increased number of BrdU-labeled nuclei. In addition, the aELH also significantly induced the 17β-estradiol and progesterone secretion in the cultured ovarian explant. In conclusion, this study demonstrated the existence of an aELH-liked-peptide in the CNS and ovary of the P. pelagicus, and it can induce the ovarian 17β-estradiol and progesterone production resulting in early ovarian maturation in this crab.