In recent years, the water filtration industry has seen an increase in planning and design of reverse osmosis plants for the treatment of seawater for potable use. In the past, conventional chemical and physical based pretreatment was utilized to reduce fouling of RO membranes. In the last decade, hollow fiber ultrafiltration (UF) membranes have been introduced into the municipal market for treatment of surface water and ground water under the influence of surface water. UF membranes have proven to be cost effective and practical for these applications. Due to the need for better pretreatment, the use of UF membranes is now being considered as a viable solution for pretreatment to seawater reverse osmosis (SWRO) plants. Although few commercial installations exist, extensive piloting of UF as pretreatment to SWRO is underway. This paper will discuss two pilot studies which were recently completed. Each study utilized hollow fiber ultrafiltration membranes for the purpose of pretreatment to RO. Both pilots were operated on Pacific Ocean seawater however, one pilot was operated on a deep basal well whereas the second pilot was operated on surface water with an open intake. Ultrafiltration membranes can provide product water with consistently low turbidity and SDI (silt density index) levels, regardless of seawater quality. UF systems require significantly less space and often have less power and chemical consumption than conventional pretreatment systems. As a result, Ultrafiltration membrane systems are a practical solution to SWRO pretreatment needs, especially for locations with limited space or variable seawater quality.