Membrane fouling simulators (MFS) are flat cell units to simulate the biofouling development of spiral-wound reverse osmosis (RO) elements. MFS units and two RO testing systems were operated in parallel, using different water types. Differences in differential pressure increase and fouling distribution between the two pilot plants were evaluated. Several RO elements and MFS units were operated with the same conditions to assess the reliability of the testing systems. In a second study, the performance of different feed spacer types assembled in full-scale RO elements was compared to the same feed spacer types assembled in the MFS unit. These studies showed that the relative biofouling impact in the MFS units was equivalent to the performance of the RO elements. Additionally, the results from the second study provide indications that a prototype 28 mil feed spacer (28 T1) may provide more significant additional biofouling resistance than any of the other spacers evaluated.