UF membranes with new Multibore design were tested in a pilot unit at the Addur SWRO Desalination Plant, Bahrain as pre-treatment for RO. The seawater at Addur is known for its extremely bad condition, mainly caused by industrial and residential waste disposals and high organic contents and bioactivity. Feed water for the pilot plant was taken-from the Addur seawater supply line (intake 1.2 kin offshore, chlorinated) directly, without making use of the existing media filtration. The existing UF/RO plant at Addur is suffering from heavy fouling and scaling problems. The pilot unit was started in March 2003. A first set of trials was done to test different operational modes and cleaning procedures under extreme fouling conditions. Based on these trials new modified UF modules were installed in March 2004 and operated until July 2005. During these 16 month of service the new Multibore UF membranes performed successfully. During summer month 2004 no membrane cleaning except normal backwash procedures with sodium hypo chloride was necessary for stable operation. The use of not chlorinated seawater as feed to the UF was tested in October 2004 and during the outage of the Addur plant February-April 2005. This water was taken by a newly installed 3" intake pipe from a location about 200 in offshore from a depth of 1.5 in only. This so called "raw beach water" turned out to be not suitable for UF filtration due to its extremely high fouling potential. The injection of flocculants into feedwater was tested to further improve the UF output and filtrate quality. The dosing of very small amounts of Fe proved to be beneficial. During summer 2005 the UF operation with Fe injection was stable with a flux of 70 l/m(2) h at a differential pressure around 250 mbar only. Filtrate SDI was always well below 3, even below 2 in July 2005. An additional RO pilot unit was installed in May 2004 to directly test the fouling potential of the filtrate from the UF pilot plant. In the RO pilot plant a permeator as it is used at the Addur SWRO plant was installed. The fouling of this permeator was remarkably lower compared to the fouling in the Addur SWRO desalination plant. This proves the high quality of the UF filtrate in addition to the low SDI values. Because of the low consumption of cleaning chemicals, low differential pressure and no need for media filtration the operating costs for Multibore UF membranes would be remarkably lower compared to the operating costs of the existing arrangement at Addur. The pilot test under the extreme seawater conditions of Addur revealed that these new UF membranes are a very promising alternative for pre-treatment for SWRO desalination plants.