Forward osmosis (FO) has proven to be a robust, low-pressure membrane separation process capable of rejecting a broad range of contaminants; thus, providing a high quality diluted brine suitable for further desalination by reverse osmosis (RO). In this study, a pilot-scale FO-RO system treated > 10,000 L of raw produced water from the Denver-Julesburg basin (Colorado) over a four-week period using commercially available FO and RO membranes. Overall, the FO-RO pilot system maintained > 99% rejection of nearly all measured ions and > 95% rejection of hydrocarbons such as semi-volatile linear aliphatic hydrocarbons and polycyclic aromatic hydrocarbons. Although the FO-RO system was able to treat raw produced water, high concentrations of organic compounds severely fouled the FO membrane and substantially reduced water flux by 68% within 21 days. Membrane degradation due to interaction between organic constituents such as aliphatic and aromatic hydrocarbons and the membrane polymer may have compromised the FO membranes, resulting in substantial increase ( x 15) in reverse salt flux within 21 days. Further investigations of membrane cleaning and pretreatment will be required in order to better understand the overall economic feasibility of treating raw produced water using FO.