Effects of synthetic oligodeoxynucleotides (ODNs) containing cytidine-phosphate-guanosine(CpG) motif(s) on nonspecific immune responses of olive flounder (Paralichthys olivaceus) and on protection against lethal infection with Edwardsiella tarda were investigated. Respiratory bunt activities of the head kidney phagocytes in the fish injected either 0.25 or 0.5 ${mu}g/fish$of ODNs containing CpG motifs (ODN 1826 and ODN 1670) were significantly higher than those injected with an ODN containing a guanosine-phosphate-cytidine (GpC) motif (ODN 1720) or with hanks balanced salt solution (HBSS, control) at 3, 5 and 7 days after injection. The serum lysozyme activities of fish injected with 0.25${mu}g$ of ODN 1826 were significantly higher than those injected with ODN 1720 or HBSS at 1 and 7 days after injection. At 7 days after injection, the group of fish injected with CpG ODNs showed higher serum lysozyme activities than fish injected with ODN 1720 or control. The group of fish injected 0.25 or 0.5${mu}g$ of CpG ODNs showed higher survival rates than those treated with GpC ODN and the control group after challenge with Edwardsiella tarda. The present study proved the ability of synthetic CpG ODN to increase nonspecific immune responses and disease resistance in olive flounder.