During microfiltration it is possible to obtain a permeate rate equal to that of the clean liquid permeation rate under certain critical conditions. This occurs when a deposit does not foul the surface of the membrane and internal deposition of material within the filter does not occur. Surface filters do not possess an internal structure, therefore, the only particulate fouling possible is that on the surface. Experiments in a stirred cell compared the surface fouling of two types of true surface filters: a commercially available track-etched filter with circular pores 10 mum in diameter and a filter with slotted pores 10 mum x 420 mum in size. When using a challenge suspension containing dilute latex particles of diameters 15 mum, and less, the circular pore membrane exhibited a critical flux at 2601 m(-2) h(-1). Under the same conditions, the critical flux for the slotted membrane was in excess of 15001 m(-2) h(-1). Surface fouling was removed by back-flushing for both filters. These results indicate that slotted pores are less likely to suffer from particles bridging the pores leading to cake deposition, or secondary membrane formation, and that further development of filters with slotted pores of smaller slot widths would be worthwhile.