Immunostimulants are valuable for control of shrimp diseases and the immunostimulatory effects of some polysaccharide additives for shrimp have been reported. In this study, the Sargassum fusiforme polysaccharide extract (SFPSE) was assessed as a feed additive when supplemented in the diet (0%, 0.5%, 1.0%, and 2.0%) for juvenile shrimp, Fenneropenaeus chinensis, in order to study the effects of SFPSE on vibriosis resistance and immune activity. Shrimp were cultured in the same pond with cages. The body weight, survival, the cumulative mortality after injection with Vibrio harveyi (30 μl V. harveyi suspension at 9.3 × 107 CFU ml−1 per shrimp), the total haemocyte counts (THCs), the protein concentration and the phenoloxidase (PO) activity in supernatant of haemolymph, the lysozyme (LSZ) and superoxide dismutase (SOD) activity in muscle of the shrimp were assayed after 14 days feeding period. The results indicated that shrimp survival under the stress of V. harveyi was affected by the dietary SFPSE. The shrimp treated with 1.0% and 0.5% SFPSE displayed significantly lower cumulative mortalities after being injected with V. harveyi suspension 24 and 30 h later, respectively, compared with that of the control. However, cumulative mortality of 2.0% SFPSE treatment was not significantly different from that of the control. There was no significant difference of cumulative mortality between 0.5% and 1.0% SFPSE treatment groups. The immune activities of the shrimp also were affected by dosage of dietary SFPSE. The THCs of the shrimp rose with increasing SFPSE dosage. The protein concentration and PO activity in supernatant of haemolymph as well as muscular LSZ activity first rose then dropped with increasing SFPSE dosage. The protein concentration in supernatant of haemolymph appeared a maximum of 167.46 mg ml−1 in 1.0% SFPSE treatment. The PO activity and LSZ activity reached the peaks as 13.20 U and 3.21 U mgprot−1 in 0.5% SFPSE treatment, respectively. SOD activity of the shrimp was not significantly affected by dietary SFPSE. It is therefore suggested that oral administration of SFPSE at an optimal level of 0.5% and 1.0% for 14 days effectively improved vibriosis resistance and enhanced immune activity of the shrimp in general.