The effects of prolonged exposure to low concentrations of herbicides on tropical periphyton (biofilm) communities are largely unknown. Tropical estuarine biofilms established in microcosms were therefore exposed to diuron (photosystem-II-inhibitor) at 2-16μgL -1 for 4weeks. The biofilms, consisting of diatoms, filamentous brown algae and cyanobacteria, developed a tolerance to diuron during this period as measured by Phyto-PAM fluorometry. Microscopy and pigment analysis revealed that this decrease in sensitivity was accompanied by a shift in species composition towards communities dominated by diatoms. The combination of techniques enabled the first identification of pollution-induced community tolerance (PICT) in tropical estuarine periphyton in response to chronic herbicide exposures. Community composition changed compared to controls at environmentally relevant concentrations of 1.6μgL -1, while development of PICT was evident at 6.5μgL -1 diuron, with no recovery (over 2weeks) in uncontaminated water, indicating chronic pollution induced shifts in community structure.