Membrane fouling was evaluated in a side stream anaerobic membrane bioreactor (AnMBR) operated for the treatment of swine manure. The AnMBR consisted of an external tubular polyethersulphone ultrafiltration membrane module (diameter = 12 mm) connected to a six-liter mixed bioreactor. The system was operated for 135 days without chemical membrane cleaning resulting in a membrane flux of 5–10 L/m 2 h. Membrane fouling was dominated by a loosely attached fouling layer, which could be removed by flushing the tubular membrane. Intensive chemical cleaning after the 135 days of continuous operation resulted in an irreversible resistance of 3×10 12 1/m, equivalent to 1.3 times the resistance of the new membrane. More frequent chemical membrane cleaning using HNO 3 could not prevent the development of irreversible fouling. Equilibrium calculations and scanning electron microscopy with energy dispersive spectroscopy demonstrated that inorganic precipitation contributed to fouling of the membrane surface and in the membrane pores.