The vasa orthologs have been used as a specific germ line molecular marker in many animal species. In this study, the expression of the vasa ortholog (povlg1) in adult and juvenile pearl oysters (Pinctada fucata) was observed by in situ hybridization. The in situ hybridization with povlg1 made it possible to detect the immature germ cells, which could not be detected by hematoxylin and eosin staining. During the reproductive season (May to July), spermatogonia, spermatocytes, oogonia, and oocytes showed povlg1 expression in the gonads of mature adult pearl oysters. By contrast, in spent pearl oysters in the nonreproductive season (September and October), only small ovoid (8.9×5.2 mm) povlg1-positive cells were observed in the base of acini. In juvenile (1 mo old) pearl oysters, a clump of germ cells first formed from several cells that were distributed symmetrically and lateral to the visceral mass. These cells then migrated to the ventrolateral periphery of the visceral mass in 2-mo-old oysters. The cells then migrated posteriorly along the periphery of the visceral mass with increasing cell numbers and size in 4-mo-old oysters. This observation of immature germ cell distribution and migration provides useful information for the control of gametogenesis and about pearl quality.