This paper presents the effect of substrate concentration on the growth of a mixed culture of algae and heterotrophic bacteria in secondary facultative ponds (SFPs) utilizing settled domestic sewage as a sole source of organic carbon. The growth of the mixed culture was studied at the concentrations ranging between 200 and 800 mg COD/l in a series of batch chemostat reactors. From the laboratory data, the specific growth rate (m) was determined using the modified Gompertz model. The maximum specific growth rate (mmax) and half saturation coefficients (Ks) were calculated using the Monod kinetic equation. The maximum observed growth rate (mmax) for heterotrophic bacteria was 3.8 day 1 with Ks of 200 mg COD/l. The mmax for algal biomass based on suspended volatile solids was 2.7 day 1 with Ks of 110 mg COD/l. The mmax of algae based on the chlorophyll-a was 3.5 day 1 at Ks of 50 mg COD/l. The observed specific substrate removal by heterotrophic bacteria varied between the concentrations of substrate used and the average value was 0.82 (mg COD/mg biomass). The specific substrate utilization rate in the bioreactors was direct proportional to the specific growth rate. Hence, the determined Monod kinetic parameters are useful for the definition of the operation of SFPs.