A growth experiment of tilapia (offsprings of the hybrid between Oreochromis niloticus and O. mossambicus) under different dissolved oxygen levels in the recirculating water system was conducted at the Fish Culture Experiment Station of the National Fisheries University of Pusan from February 4 to March 5, 1986. Six tanks with a capacity of $1.8m^3$ of water each were used under the same condition of water parameters except for dissolved oxygen levels which were designated to maintain at 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mg/$iota$. Each tank was stocked with 90 kg of fish each averaging 64 to 69 grams. The average water temperature during the course of the experiment was $22.5^{circ}C$. The results obtained are summarized as follows: The food conversion efficiencies were very good, being 1.05-1.11 at 3.5, 3.0, 2.5 and 2.0 mg/$iota$ DO levels without any significant differences among them, but at 4.0 mg/$iota$ the F. C. was 1.39 and at 1.5 mg/$iota$ it was 1.61 being very poor compared with the others. The daily growth rate performance was best at 3.5 mg/$iota$ dissolved oxygen level followed by 3.0 and 2.5 mg/$iota$ with slight differences while at 4.0 and 2.0 mg/$iota$ DO levels the growths were significantly poor, and at 1.5 mg/$iota$ DO level it was extremely poor. In 1.5 mg/$iota$ group, the fish did not accept feed vigorously and after feeding the fish usually concentrated around the inflow point showing oxygen deficiency response. While at 4.0 mg/$iota$ high feeding rates tended to waste significant amounts of feed while eating and led to water deterioration, and above these levels the results is considered to lead to a waste of energy with uneconomical performance. On the other hand, at and below 2.0 mg/$iota$ DO level the tilapia certainly showed a poor growth performance. The experiment indicates that the DO range of 2.5$~$3.5 mg/$iota$ is the optimum level for the good growth performance.