An extract of ulvan obtained from the green alga Ulva clathrata was lyophilized and confirmed by Fourier transform infrared spectrometry (FTIR) and H-1-nuclear magnetic resonance (NMR). The percentage of sulfate groups calculated by elemental quantitative analysis in the lyophilized ulvan was twice (8 %) than that of dry algae (4 %). The dietary effect of ulvan on hematology, immunology, and growth was evaluated in tilapia (Oreochromis niloticus). The fish (25 +/- 3 g) were fed a commercial diet (negative control group), and the diet was supplemented with three levels of ulvan (0.1, 0.5, and 1 % kg(-1) feed). Additionally, a positive control with yeast (Saccharomyces cerevisiae, 2 % kg(-1) feed) was included. During a trial period of 90 days (60 days of treatment + 30 days of suspended treatment), hematocrit (Ht), hemoglobin (Hb), red blood cell (RBC) count, white blood cell (WBC) count, total serum protein (TP), albumin (Al), globulin (Gb), and phagocytic activity (Pa) were measured. After 15 days of treatment until the 60th day, the Pa was increased (P < 0.001) in fish with diets containing 0.1, 0.5, and 1 % ulvan. An increase (P < 0.001) in the WBC count was observed at day 30 in fish with diets containing 0.1, 0.5, and 1 % ulvan; which maintained for 15 days in 0.5 and 1 % ulvan after their dietary suspension. There was no significant difference (P < 0.05) in growth performance among fish for any of the diets tested. This is the first report of an increment in the immune response by a dietary supplementation with ulvan from U. clathrata in the Nile tilapia. These results demonstrate the potential activity of ulvan on at least two mechanisms of the immune response in tilapia and the time over which the activity is sustained in the organism after suspension of supplementation.