In spite of the great advances in increasing the reliability, and technical and economic viability of seawater desalination by reverse osmosis (RO) made over the past decade, its commercial application is still limited. Unit design is highly dependent on the quality of the feedwater and pretreatment method used to safeguard membrane performance. Work is in progress at Doha Research Plant (DRP), in Kuwait, to determine the performance of two seawater RO units under the prevalent seawater conditions. The RO units have st total capacity of 300 x 2 m(3)/d and are fed with beachwell seawater with a silt density index (SDI) of around 2, without any further pretreatment. The two RO plants are identical in all operational aspects, but differ in membrane configuration, i.e., twin hollow fine fiber and spiral wound. This paper discusses the analytical performance of the two types of membranes using a single-stage system. The paper covers one year of performance data and discusses the technical parameters of water productivity and permeate quality. It also covers the evaluation of membrane performance, system availability and operational problems encountered.