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A membrane bioreactor (MBR) pilot plant was operated in two subsequent experimental periods (namely, Periods I and II) with different start-up and sludge withdrawal strategies to study its peculiar biokinetic behavior by using respirometric techniques. Two extreme operational conditions were chosen to investigate the different biomass activity under dynamic or pseudostationary conditions during and after the start-up phases. Particularly, the MBR pilot plant was operated with the same volumetric loading rate (VLR) and permeate flux but differently managed during the start-up phase. In Period I, the MBR pilot plant was started up without sludge inoculum and operated without sludge withdrawals; on the contrary, in Period II the MBR pilot plant was started up with sludge inoculum and operated with regular sludge withdrawals. The obtained results highlighted that the different start-up strategies significantly affected the value of the yield coefficient, the storage phenomena, the decay coefficient, and the maximum respiration rates of heterotrophic species. More specifically, both experimental periods underlined interesting aspects related to the bacterial behavior under significant stress conditions occurring during a non-steady-state (start-up) phase. [ABSTRACT FROM AUTHOR]