Clownfish, Amphiprion ocellaris (2.4 cm) were cultured in recirculation system, volume of 50 L and density of 20 fish per tank. Six levels of mannan oligosacharide (MOS). including 0.05 percent, 0.10 percent, 0.15 percent, 0.20 percent, 0.25 percent and 0.30 percent were supplemented to the control diet and fed to the fishfor 10 weeks. Collected data included ratio of internal and external intestinal perimeter of the gut (PR), muscle protein content and total blood cell count. Result showed that the PR was highest in the fish fed by 0. 10 percent and 0.15 percent MOS and lowest in two groups of fish fed by control and 0.3 percent MOS in their diet (P 0.05). After 4 weeks of diet feeding, protein, content in the fish among the treatments rangedfrom 12.85 percent P to 15.53 percent P. Those values seemed to increase at week 10 in compared to week 4, however
there was no sigl1ificant difference in protein content among the treatments at either week 4 or week 10 measurements. Total blood cell cdunt of fish at week 10 were highest in the group of fish fed on 0.05 percent MOS (26.22 x 10 exponent 5 + or - 2.80 x 10 exponent 5 cells/mm3, followed by the groups of fish fed 0.25 percent and 0.1 percent with blood count being 21.54 x 10 exponent 5 + or - 3.65 x 10 exponent 5 cells/mm3 and 20.73 x 10 exponent 5 + or - 7.88 x 10 exponent 5 cells/mm3. Respectively, the lowest blood cell count was in the fish fed by 0.15 percent MOS (11.02 x 10 exponent 5 + or - 2.00 x 10 exponent 5 cells/mm3). There was a statistically significant difference between blood cell count of fish fed by 0.05 percent MOS in compared to fish fed by control diet and 0.15 percent MOS (P 0,05).