Abstract- Study was conducted to evaluate effect of C:N ratio on growth and survival rate of spotted scat fingerlings in biofloc system. Spotted scat with initial size of 0.149±0.03 g was reared in 100 L tank at salinity of 5 ppt. Experiment includes 4 treatments with C:N ratios: (i) no molasses addition (control), (ii) C:N=10, (iii) C:N=15, (iv) C:N=20. Each treatment was triplicated, and experiment lasted for 45 days. Spotted scat was reared at density of 500 ind/m3 and molasses was supplied every 3 days. Treatment with C:N=15 has the highest floc volume (7.33±2.08 ml/L) and total bacteria (650±122×104 CFU/mL), the lowest Vibrio rate (0.21±0.07%). Fish reared at C:N=15 has highest average weight (0.97 g/ind), growth rate in weight (0.020 g/day, 4.07 %/day), survival rate (86.0±2.00%), biomass (416±35.5 g/m3), it was also the lowest FCR (1.39±0.15) and they all differed significantly with the other treatments (p Index Terms- Scatophagus argus, biofloc, spotted scat, molasses, C:N ratio, growth, survival rate