Uptake and excretion of 15N by tilapia was evaluated in simulated bio floc technology (BFT) tanks. Bio floc suspension (suspended solids in the range of 200 mg l−1) was enriched in respect to 15N by adding tagged ammoniumsalt and starch, to ensure a complete immobilization of ammoniumin the bio flocs. Fish were held for 14 days in tanks,when the only feed sourcewas the flocs. Nitrogen species in thewater, aswell as 15Nenrichment in the suspended matter and in fish muscle were monitored at frequent intervals during this period. The system under study underwent a series of processes, including the uptake by fish of suspended organic nitrogen, excretion of tagged and non tagged nitrogen by fish, microbial degradation of the bio flocs and the opposing process of new bio flocs production using the excreted nitrogen.