Microfiltration of a double-chain cationic surfactant, dioctadecyldimethylammonium chloride, in water has been studied using 1 or 0.2 mum track-etched polycarbonate membranes. A computer-controlled filter rig was used, in which the process conditions of crossflow velocity u, transmembrane pressure drop DELTAp and temperature T were changed independently. In addition, permeate flux J and in some cases surfactant concentration in the feed cf and permeate c(p), were monitored during filtration. The effects of process and material variables on the permeate flux decay, steady-state permeate flux J(infinity) and the permeate surfactant concentration were assessed. It was found that fouling of the membrane by surfactant is very rapid (within tens of seconds) although high crossflow velocity, large membrane pore size and low feed surfactant concentration reduced the rate of surfactant deposition. Steady-state permeate flux increases significantly with crossflow velocity provided the surfactant is in a rigid chain (gel) lamellar dispersion state (T < 48-degrees-C). The steady-state permeate flux J(infinity) decays linearly with In (c(f)) provided the feed concentration is less than the so-called gel concentration (c(g)) which is obtained by extrapolating the linear portion of the curves J(infinity) vs In (c(f)) so that c(f) = c(g) when J(infinity) = 0. Although the gel concentration, which is independent of pore size and the process variables, is found to be 20 g l-1, experiments conducted at feed concentration well above c(g) still yield a reasonable permeate flux, albeit at a reduced level. Surfactant rejection studies indicate that 0.2 mum membranes yield lower rejection than 1 mum membranes while rejection is greater at 30-degrees-C than at 60-degrees-C. Other process variables do not have any significant influence on rejection. The effectiveness of a membrane and the processing conditions can be quantified by considering the variation of J with c(p)/c(f). It is found that a 1 mum membrane is more effective than a 0.2 mum membrane, crossflow velocity increases the effectiveness of the process, and that temperature has no significant influence.