There is disclosed, in one aspect, an improved highly asymmetric membrane comprising a skin and a porous asymmetric support. The skin contains pores which have an mean pore diameter of from about 0.005 to about 3.0 microns and the asymmetric support comprises a reticulated structure which contains pores which have pore diameters of from about 10 to about 20,000 times as large as the pore diameter of the pores of the skin. The membrane has a bulk porosity greater than about 70%. In another aspect, there is disclosed a process for preparing these membranes. This process comprises casting a polymer dope while the dope is in an unstable liquid dispersion condition. The concentration of polymer in the polymer dope should be high enough to produce a coherent membrane yet low enough to form a substantially all-reticulated structure within the asymmetric support. The membranes are characterized by high flow rates, excellent retention properties, and improved resistance to fouling.