Tilapia aurea and T. mossambica fry were stocked in a flowing system at varying stocking densities (5,10,20,30,40,50 and 60 fish/tank in 60 liters of water) in an intensive tank culture system. The fish were maintained for 1I5 days on commercial pelleted feed. In terms of length increase, weight gain, condition, total yield, and food conversion rates, T. aurea performed significantly better than T. mossambica at all stocking densities. The T. mossambica experienced much higher mortality and reduced growth due to an auto-immune reaction related to stocking density, which effectively limits the density at which this species can be stocked. Hypersensitivity reactions began when the biomass reached about 20g/liter with a turnover time of one-half hour, although further research is needed to completely quantify their crowding tolerance limits. T. aurea were still growing and feeding vigorously at termination of the experiment (with a maximum biomass of 66.2g/liter) and probably could have continued to grow to even greater biomass. T. aurea is preferable to T. mossambica in intensive fish culture or in tilapia polyculture due to the former's higher yields and greater resistance to crowding.