ABSTRACT. – We studied the adverse effects of triclosan, a widely used biocide commonly reported in surface waters, on the structure and function of benthic diatom communities. Laboratory- grown biofilms were exposed (i) to chronic contamination by increasing concentrations of triclosan and (ii) to a short-pulse of sublethal triclosan concentrations followed by a 2-week recuperation period. The first experiment was performed using 6 nominal concentrations ranging from 0.05 to 500 μg/L triclosan to obtain the concentration – effect relationships for benthic diatom communities. Here effects at the highest triclosan concentration in the diatom community consisted of a 63 % increase in diatom mortality, with respect to control conditions. The second experiment aimed at determining the long-term effects of the toxicant and biofilm recuperation after addition of 60 μg/L triclosan for 48 h exposure. After two weeks the sublethal pulse had caused a decrease in diatom growth rates and a significant delay in the exponential phase of growth. The triclosan pulse provoked a decrease in diatom species richness and diversity. The diatom communities were dominated by six species, with Achnanthidium minutissimum being highly preponderant, and variations were not large enough to provide information about sensitivities / tolerance to triclosan in different species.