Wastewaters containing organic compounds have been treated using extractive membrane bioreactors (EMBs). During treatment, a biofilm normally develops on the surface of the membrane, on the biological side. This study investigates the dynamics of biofilm growth in an EMB exposed to an alternating sequence of organic compounds. Microbial dynamics of both suspended and attached cultures were investigated experimentally in a single-tube extractive membrane bioreactor (STEMB), which comprised a continuous stirred-tank bioreactor (CSTB) coupled to eight single-tube extractive membrane modules (STEMMs) via a recirculating biomedium. A model microbial culture consisting of a Burk-holderia sp, strain JS150 (ATCC No. 51283), able to degrade monochlorobenzene, and a Xanthobacter autotrophicus sp. strain GJ10 (ATCC No. 43050), able to degrade 1,2-dichloroethane, was used. Both microbial strains exhibited exclusive degradative capabilities. The CSTB was monitored by quantification of individual strains and by product and organic compound evolution. To investigate the biofilm growth dynamics, eight STEMMs were run in parallel with the same operating conditions. Every week, STEMMs were stopped for biofilm analysis and the organic compound in the wastewater was changed. Biofilm growth was investigated by quantification of individual strains, by evaluation of the;overall biofilm growth, and by microscopic analysis. A biofilm composed of both strains was developed and maintained during the whole experiment in the STEMMs. The biofilm that developed on the membrane improved the response of the system to changes in the wastewater.