A marine microalga Gyrodinium impudicum strain KG03 produced sulfated exopolysaccharide designated as p-KG03, which showed a strong antiviral activity against encephalomyocarditis virus (EMCV). To optimize culture conditions for the production of p-KG03, mineral salts, vitamins, plant growth hormones, temperature, pH and light conditions were examined. From this study, M-KG03 medium for the maximum production of p-KG03 was suggested as follows; NH 4Cl 75 μM, NaH 3PO 4 200 μM, NaHCO 3 50 μM, Na 2SO 4 10 μM, FeCl 2·6H 2O 10 μM, MnCl 2·4H 2O 0.1 μM, vitamin B 12 0.75 μg, naphthalene acetic acid (NAA) 7.5 μg and myo-inositol 200 mg per liter of aged sea water. The optimal temperature and pH were 22.5 °C and 8.0, respectively. The optimal light conditions of intensity and period were 150 μE m −2 s −1 and 16:8 h light:dark cycle. Finally, the cell growth and p-KG03 production were measured in one liter of M-KG03 medium with 1% CO 2 and 50 ml min −1 of airflow using two liters airlift balloon type photobioreactor (ABTPR). At these optimal conditions, p-KG03 production and cell growth were 134.6±5.9 mg l −1 and 123,076±1,597 cells ml −1, respectively, representing a 7.7 and 5.1 times compared with f/2 medium with Erlenmeyer flask culture (p-KG03 production 17.5±1.3 mg l −1 and cell growth 24,311±1,291 cells ml −1).