Long Beach Water Department (LBWD) has embarked on a three-phase research and demonstration program to develop and implement a new, lower cost, seawater desalination process, using membrane technology, to supplement their current imported and local water supplies. Water conservation and water recycling efforts alone at Long Beach are not expected to offset the increasing demands for new potable water supplies and potential reductions and restrictions to the present imported water supplies. Located in coastal Southern California, Long Beach has access to Pacific Ocean seawater and saline groundwater of seawater quality as potential new water sources. The three-phase seawater desalination program of the Two-stage Nanofiltration Membrane Desalination Process includes Pilot Testing (Phase 1), Prototype Seawater Desalination Testing Facility Design, Construction and Operation (Phase 2) and Demonstration Plant Design, Construction and Operation (Phase 3). The Phase 1 pilot-scale testing, which began in 2001, consisted of 4-inch (102-mm) diameter nanofiltration (NF) membranes in a proprietary two-stage (two-pass) configuration at a capacity of 9,000-USgallons per day (gpd) [34 m3/day]. During this pilot testing, the method demonstrated that it can achieve treated water quality equivalent to single-pass seawater reverse osmosis (SWRO) desalination process at lower operating pressures and energy cost. Due to the success of the pilot-scale testing and promise of this desalination process for the full-scale, LBWD has applied for a patent on this innovative method and is proceeding to the next phase of the program. LBWD has two research partners for the program, the United States Bureau of Reclamation (USBR) and the Los Angeles Department of Water and Power (LADWP). Phase 2 consists of testing full-scale 8-inch (203-mm) diameter NF membrane elements in a 150,000-gpd (568 m3/day) prototype plant and having a side-by-side comparison with a 150,000-gpd (568 m3/day) conventional seawater reverse osmosis (RO) membrane desalination plant. If Phase 2 Prototype Plant testing proves successful, LBWD will proceed to a full-scale demonstration plant, with a capacity of up to 10- million USgallons per day (37,850 m3/day). This paper summarizes the Phase 1 pilot-testing, details the current Prototype Seawater Desalination Testing Facility phase, and briefly describes the future Demonstration Plant phase (Phase 3). Includes 9 references, tables, figures.