Fiber glass-reinforced plastic immersed in an experimental estuarine mesocosm fouled at estimated rates of 0.5, 5.5, and 18.8 ng (wet weight) 1/sq mm day over days 0 to 2, 2 to 6, and 6 to 14, respectively. Protists, dominated by diatoms, which developed between days 3 and 6 and covered 90% of the undisturbed surface in 2 weeks, were effectively removed by twice-weekly brushing of the surface to maintain an immature 3-day bacterial film which covered 12% or less of the surface and had a biomass 3 orders of magnitude smaller than surfaces with 2 weeks' unrestricted fouling. Direct brushing of the fiber glass-reinforced plastic tank walls of experimental estuarine mesocosms minimized the 'wall effect' by keeping a surface that maintained a low biomass of a slowly accumulating bacterial film rather than a surface which supported the more rapid accumulation of protists which in turn may induce the settlement of invertebrates and macrophytes.