A five-level-three factor central composite rotary design was employed to find out the interactive effects of three variables, i.e. static magnetic field intensity, concentration of NH3-N and initial pH on polyhydroxyalkanoates (PHA) production by activated sludge under the aerobic dynamic feeding (ADF) technique. Response surface methodology (RSM) was utilized for process optimization and a second-order polynomial equation was obtained by multiple regression analysis. A yield of 49.5% of dry cell weight (dcw) was achieved at optimized conditions, i.e. magnetic field 11mT, NH3-N 4.8 mg l(-1) (C:N = 60:1)and initial pH 9.0. (C) 2008 Elsevier Ltd. All rights reserved.