The effects of fennel essential oil (Foeniculum vulgare) at 5 levels of 0 (control), 100, 200, 400 and 600 mg per kg. of diet on growth performance, survival, body composition and hematological parameters of the Caspian kutum fry, Rutilus frisii kutum, were investigated in a 60-days feeding trial. No significant differences in the growth related parameters and survival were observed among treatments (p>0.05). The highest level of fat was observed in 100 mg fennel essential oil (p<0.05). The highest level of white blood cells was observed in 100 mg and the highest levels of red blood cells, hematocrite and hemoglobin were observed in 400 mg; the lowest level of MCV was observed in the control. No significant differences were observed in other parameters such as MCH and MCHC. In conclusion, fennel essential oil had no significant effect on the growth related parameters of the Caspian kutum fry, but 100 mg of fennel essential oil/kg diet would play an important role in promoting immune system of the fish by increasing the white and red blood cells. [ABSTRACT FROM AUTHOR]