This study evaluated the effects of adding sea mustard Undaria pinnat 4/ida glycoprotein to the diet of juvenile olive flounder Paralichthys olivaceus on its growth, and levels of insulin-like growth factor I (IGF-l), IGF binding proteins (IGFBPs), and interleukins. Three experimental diets (U0, UO.5, and U 1.0) were formulated that contained different amounts of an extract of U. pinnat4flda (0, 0.5, and 1.0%, respectively), Experimental groups were established in triplicate (30 fish/group) and fed for 12 weeks, The experimental group fed 1.0% added U pinnatjflda glycoprotein had the greatest rate of weight gain, which differed significanily from the other experimental groups. SDS-PAGE of the plasma IGF-I and muscle protein showed that the experimental groups taking U pinnatjflda glycoprotein had significantly more IGF-1 and a ca. 200kDa protein, as compared to the control group. In addition, the amount of IGF`BP-3 at ca. 43kDa increased in the group given the U. pinnatiiflda extract, as compared to the control group. The interluckin-2, -4, -6, and-12 levels paralleled the level of growth factor in the groups given the U. pinnat i/Ida extract. In conclusion, supplementing the diet of olive flounder with U. pinnatifida glycroprotein improved its growth and immunity.