For the prediction of the behavior of municipal wastewater treatment facilities, the Activated Sludge Model No.1 (ASM1) is often used. Application of the ASM1 required the determination of kinetic and stoichiometric parameters and wastewater composition. The aim of this study is to determine the heterotrophic growth yield (YH) using, which was not determined before in Egypt, respirometric test method. Respirometry, which consists in the measurement of the oxygen uptake rate, is a method commonly used to determine kinetic and stoichiometric parameters of biological processes. YH was measured seven times with different initial substrate-to-biomass (S0/X0) ratios. It was found that the value of YH for the wastewater obtained from Al Qenayat wastewater treatment plant was 0.55 mg COD biomass/mg COD substrate, which is somewhat lower than ASM1 default value.