The induction of cellular and humoral immunity and cytokine gene expression by synthetic CpG oligodexoynucleotides (CpG-ODNs) has not been investigated systematically in olive flounder Paralichthys olivaceus in vivo. We optimized the proper concentration of CpG-ODNs using an in vitro assay for the superoxide anion $(O_2^-)$. CpG-ODNs induced $O_2^-$ and nitric oxide (NO) production, lysozyme activity, and the proinflammatory cytokine gene expression of $IL-1{eta}$ and $TNF-{alpha}$ in olive flounder significantly in vivo, whereas non-CpG-ODNs did not produce these effects or produced them to a lesser extent. This implied that CpG-ODNs could stimulate cellular and humoral immunity and cytokine gene expression in olive flounder. This is the first evidence of NO production and the first study on the mRNA expression of the proinflammatory cytokine genes $IL-1{eta}$ and $TNF-{alpha}$ in olive flounder in response to CpG-ODNs. Comparison of the variation in NO production and lysozyme activity to that of other studies led us to postulate that a group-specific difference exists in the immune responses of olive flounder against CpG-ODNs. Furthermore, the detailed immunostimulatory spectrum of CpG-ODNs in olive flounder could be a useful index with which to analyze the effect of CpG-ODNs against the challenge test prior to field applications.