In this paper a new dimension is given to the concept of membrane fouling. In the already known fouling mechanisms for dead-end operated ultrafiltration membranes fouling is considered a radial phenomenon. However, fouling also appears to have longitudinal effects. Experiments were performed with a lab-scale test installation under constant flux conditions with demiwater that contained ferric hydroxide flocs as fouling components. The experiments showed that during operation capillaries became blocked by fouling plugs resulting in capillaries that were filled completely with solids. The presence of blockages, especially at the concentrate side of the capillaries, could not be detected by measuring the clean water resistance. However such blockages did result in an increased forward flush pressure. A combination of the clean water resistance and the forward flush pressure is suitable for determining the fouling of a membrane and the effectiveness of a cleaning procedure.