We have observed by flow cytometric analysis that a small but consistent percentage of circulating hemocytes in phases S, G2 and M of the cell cycle can be detected in Penaeus japonicus. Significantly increased percentages of proliferating hemocytes (approximately three-fold increase) were observed after stimulation by lipopolysaccharide (LPS), p43 (an immunosuppressive lymphocyte mitogenic protein produced by Candida albicans) and a combination of LPS and p43. Moreover, an approximately six-fold increase in the percentage of proliferating hemocytes was also observed in animals infected with Fusarlum opp., as compared to non-infected shrimps. Furthermore, [3H] thymidine uptake in circulating hemocytes was 26 times greater in LPS stimulated than in non-stimulated shrimps. The present study suggests that circulating hemocytes of the shrimp P. japonicus can divide in vivo and that proliferation can be increased significantly after mitogenic or infectious stimulation.