White shrimp Litopenaeus vannamei immersed in seawater (35‰) containing fucoidan at 0 (control) and 200 mg/l for 3 h were exposed to 5 mg/l ammonia-N (ammonia as nitrogen), and immune parameters including hyaline cells (HCs), granular cells (GCs, including semi-granular cells), total hemocyte count (THC), phenoloxidase (PO) activity, and respiratory burst (RBs) were examined 6~24 h post-stress. The GCs, PO activity and RB of shrimp immersed in seawater containing fucoidan at 200 mg/l returned to original values earlier at 12~24 h post-stress, whereas in control shrimp they did not. The HCs and THC of shrimp immersed in seawater containing 200 mg/l fucoidan were significantly higher than those of control shrimp at 12~24 h post-stress. It is concluded that white shrimp immersed in seawater containing fucoidan exhibited a capability for maintaining homeostasis by regulating their cellular and humoral immunity against ammonia stress as evidenced by earlier recovery in immune parameters.