The objective of this research is to study the effect of the substrate/biomass ratio (F/M) on the performance of two immersed membrane bioreactors operating, respectively with a sequential (SMBR) or a continuous (CMBR) mode. A synthetic organic substrate was selected to analyse the mixed culture behaviour and an average 1 gCOD L-1 day(-1) volumetric organic load was imposed in both MBRs. Each system worked during a 150 days operational period without any sludge extraction. In both cases, the global performance was relevant due to the porous membrane selectivity and to the high reactors biomass concentration. In fact, the COD removal efficiency was more than 94%, and the permeate free from suspended solids with a turbidity lower than 0.5 NTU. Due to non-continuous conditions, the SMBR system appeared hardly less successful than the CMBR system, with a COD effluent concentration and a sludge production, respectively equal to 125 mg COD L-1 and 0.092 g CODp g CODT-1 which are approximately twice higher than in CMBR system. These differences are presumably linked with the very high instantaneous F/M ratios imposed on the SMBR system during the feed periods (from 12 g COD g MLVSS-1 day(-1) at the beginning of the experiment to 2.5 g COD g MLVSS-1 day(-1) at the end of the operation). These brief high F/M ratios could favour a cell replication behaviour which is generally linked to important microbial soluble compounds production (soluble COD) and unfortunately associated with an accumulation of products coming from cell lysis (sludge production). Despite these observations, the SMBR system appears as an appropriate solution in case of non-continuous wastewater productions as often observed on industrial sites. (C) 2007 Elsevier B.V. All rights reserved.