An anaerobic membrane bioreactor (MBR) was operated for 367 days to treat domestic wastewater originating from the Sfax wastewater treatment plant. The results showed that the MBR was not very efficient because of the considerable fluctuations in the domestic wastewater composition and the presence of toxic compounds resulting from the industrial effluents in the Sfax region. The acute toxicity tests used for this purpose were the LUMIStox test with the luminescent marine bacterium Vibrio fischeri and the phytotoxicity test with Lepidium sativum. The results of these tests showed that the Sfax wastewaters were highly toxic. The rate of inhibition of Vibrio fischeri ranged from 11.7% to 95%. In all the samples tested, the germination index was less than 15%, as compared with the 100% germination index obtained with distilled water. In addition, the fact that the tendency towards microtoxicity and the chemical oxygen demand (COD) increased during the day, when industrial activities are most intense, confirms that these wastewaters are particularly toxic during the day.