Numerous tests around the world have proven that ultrafiltration (UF) provides optimum pretreatment for seawater desalination based on reverse osmosis membranes (SWRO). Ultrafiltration will remove all suspended solids and will provide a substantial reduction in micro biological activities. Plugging of RO spacers is completely eliminated and the RO cleaning frequency can be substantially reduced. The main obstacle against the use of ultrafiltration membranes for SWRO pretreatment has always been the higher operating cost of ultrafiltration when being compared with conventional pretreatment. Up until now this higher cost has prevented implementation of UF in all but a few SWRO plants. A new membrane has been designed with the aim of tailoring it towards lowest total cost of ownership. Typical operating conditions have been used to quantify the following parameters for a potential large scale desalination system (UF + SWRO): amortization of investment in UF membranes and equipment; operating costs of the UF system; reduction in operating costs of the SWRO desalination plant, when being compared against a conventional pretreatment + SWRO system; the increased output of the SWRO desalination plant due to higher availability and shorter construction time. The total cost of ownership of a UF based SWRO desalination plant has been determined (expressed in US$/m(3) of water produced). Taking all factors into account the total cost of ownership of a dual membrane desalination plant (Seaguard UF + SWRO) will be 2-7% lower than the total cost of ownership of a SWRO plant based on conventional pretreatment.