Membrane fouling during filtration of lager beer with microsieves was studied through in-line microscope observations. It was observed that the main fouling was caused by micrometre-sized particles, presumably aggregated proteins. These particles formed flocks covering parts of the membrane surface. Most of the flocks could be removed by a strong temporary increase in crossflow. Underneath the flocks a permanent fouling layer was formed inside the pores. This made frequent removal of the flocks crucial in delaying the process of permanent in-pore fouling. Besides the fouling process the influence of pore size on permeate flux and turbidity was investigated. Centrifuged beer appeared to give a significantly clearer permeate than rough beer. For centrifuged beer and a microsieve with a pore diameter of 0.55 mum a haze of 0.23 EBC was obtained during 10.5 h of filtration at an average flux of 2.21 x 10(3) l/m(2) h. For a sieve with slit-shaped perforations of 0.70 mum x 3.0 mum a haze of 0.46 EBC was obtained during 9 h of filtration at an average flux of 1.43 x 104 l/m(2) h. This flux is more than two-orders of magnitude higher than is commonly obtained with membrane-filtration of lager beer. Concentration of the beer by a factor of 12 hardly influenced the magnitude of the flux.