Produced water from gas and oil operations is purportedly the largest single source of waste generated in the U.S., with an annual production rate of more than 2.7 bil t (3 bil ton). Results are presented from bench-scale pilot tests of membrane ultrafiltration of produced water obtained from three operating oil and gas wells. Ultrafiltration (UF) reduced concentrations of grease and oil in the three produced waters evaluated in this work to below current and anticipated regulatory limits. Permeation flux varied from one produced water to another. Because membrane cost is influenced by permeate flux, generalizations regarding the economic or technical feasibility of UF treatment of produced water based on the limited number of laboratory and field tests performed to date do not appear to be warranted.