An aerobic membrane bioreactor (MBR) at complete biomass retention was studied over a period of time under starvation conditions. Kinetic parameters were determined in a no-feed batch test. The decay rate of activated sludge, k(d) = 0.05 d(-1), was determined by tracking the decrease of MLSS. The ratio of MLVSS/MLSS was in the range 0.76-0.85. The pH values were between 7.02 and 8.23. As a function of different initial concentrations of MLSS, specific nitrification rates q(N), decreased from 4.23 to 0.02 mg-N/(g MLVSS d) and specific biodegradation rates q(b) increased from 0.23 to 1.90 mg-COD/(g MLVSS d). From experimental data the kinetic constants for respiration, which followed Monod kinetics, were determined as qO(2max) = 9.8 mg-O-2/(g MLVSS h), K-x = 2.9 g/dm(3). Additionally, a linear correlation between MLSS and mean floc size was found to exist during the biodegradation process.