Periphyton communities, despite being relatively easy to investigate compared to other communities (i.e. macroinvertebrate or fish communities), provide a perspective of community ecotoxicology. In contrast to single-species tests, ecotoxicological studies with periphyton involve a higher degree of ecological realism, allowing the simultaneous exposure of species from phylogenetically distant groups (algae, bacteria, fungi and protozoa), thus allowing the evaluation of direct and indirect toxic effects [1]. Periphyton communities have an important role as primary producers and as a food resource for freshwater grazers, so toxicant effects could be perceived at higher levels of the ecosystem [2]. At the same time, in natural conditions grazers may have a top-down control on periphyton affecting its biomass and metabolism. We present the results of a microcosm experiment aiming to assess the effects of an acute exposure of freshwater periphyton communities under grazing pressure (the snail Radix ovata) to triclosan (22 μg /L). Triclosan, as bactericide, is very toxic to the bacterial component of periphyton, with NEC of 0.21μg L-1 [3]. Moreover, lower but also direct effects on the algal component [4,3], the architecture of the whole periphyton [5], and to their grazers [6,7] are also expected under the presence of triclosan. By using a set of endpoints covering the structure and function of the algal component, functional attributes of the whole periphyton and their antioxidant enzyme activities, we evaluated direct and indirect effects of triclosan on periphyton with and without grazing pressure. This experiment illustrates how periphyton can be used as community ecotoxicology model to assess the chain of effects that a toxicant may have on aquatic systems.