To provide design data applying to the treatment of wastewater from dispersed dye factories, an experimental study was undertaken using bench-scale completely mixed activated sludge reactors fed with dispersed dye factory effluent as a substrate. Kinetic coefficients were determined at different hydraulic retention times and compared with published data for other substrates. The results showed that when biochemical oxygen demand (BOD) was utilized as a measure of substrate concentration, the values obtained for the substrate removal rate coefficient k =0.00063 L/mg BOD-hr, sludge yield coefficient Y=0.26 kg MLVSS yield/kg BOD removed, and sludge endogenous decay coefficient Kd=0.003 I/day.