A rotating biodisc contactor (RBC) operating continuously was used for biological treatment of saline wastewater. Effect of liquid phase aeration and salt concentration on COD removal efficiency and rate were investigated. A synthetic wastewater composed of diluted molasses, urea, KH2PO4, MgSO4 and various concentrations of salt (1-5% NaCl) was used throughout the study. A fifty percent mixture of salt tolerant Halobacter halobium and activated sludge culture was used as inoculum. Liquid phase aeration improved both COD removal rate and efficiency significantly as compared to unaerated liquid phase. A mathematical model describing the effects of COD and salt concentrations on system's performance was developed. The model parameters were determined by using the experimental data for both cases.