Penaeidins are a unique family of antimicrobial peptides specific to penaeid shrimp and have been reported mainly function as anti-bacterial and anti-fungal. In order to investigate whether penaeidins could also respond to virus or not, we examined the effect of WSSV on MjPen-II (penaeidin in kuruma shrimp, Marsupenaeus japonicus) expression. In the control group, MjPen-II transcript level can be detected in almost all test tissues but was expressed most strongly in hemocytes. After WSSV infection, MjPen-II transcript level was significantly downregulated in hemocytes. Moreover, the proportion of MjPen-II+ hemocytes was not significantly different between non-infected and WSSV-infected shrimp, but the number of MjPen-II+ highly expressing hemocytes decreased after infection. In addition, MjPen-II was observed in the cytoplasm of granule-containing hemocytes. These results suggest that WSSV suppresses MjPen-II expression in hemocytes.