The aim of this study was to quantitatively analyze a pattern of proliferation of gonial cells and to demonstrate neural involvement in spermatogonial proliferation of the scallop by the in vitro experiment. Immunocytochemistry for incorporated BrdU was used to identify mitotically active gonial cells. The pattern of proliferation of gonial cells was divided into two phases: phase I; oogonia and spermatogonia slowly proliferate through the growing stage: phase II; oogonia develop into oocytes and spermatogonia start to proliferate rapidly from the mature stage through the spawning stage. The neurons detected with anti-mammalian (m)GnRH antibody were distributed sparsely in the pedal ganglion and predominantly in the cerebral ganglion of both sexes at the growing stage. The extracts from the cerebral and pedal ganglion (CPG) of both sexes collected at the growing stage promoted proliferation of spermatogonia in the in vitro culture of the testicular tissue as well as mGnRH. However, CPG extract had no effect on oogonial proliferation. The increased mitotic activity induced by CPG and mGnRH was abolished by the addition of mGnRH antagonists and anti-mGnRH antibody, suggesting that the spermatogonial proliferation is regulated by GnRH-like peptide in CPG of the scallop. The same mitotic activity as CPG extract and mGnRH was observed in the hemocyte lysate, but not in the serum. These findings suggest that the spermatogonial proliferation at phase II in the scallop may be under the neuroendocrine control by GnRH neuron in CPG.