Biofilms are excellent bioindicators for water quality assessment because of their ability to integrate contamination, and their position at the base of the trophic chain in aquatic environments. Pesticides are ubiquitous in aquatic environments and can constantly interact with aquatic organisms, including those that make up biofilms, at fluctuating concentrations. The aim of this study was to describe pesticide behaviour in biofilms. Previous research highlighted that contaminant sorption was not always linear, but no study considered organic bioaccumulation isotherms and toxic impacts to biofilms concurrently. In order to characterize pesticide sorption isotherms in biofilms and the mechanisms involved in the uptake process, we simultaneously assessed bioaccumulation and toxic impact of diuron (a photosynthesis inhibiting herbicide) at the water-biofilm concentrations equilibrium. Mature biofilms grown on glass slides during one month were subsequently exposed in channels to 7 increasing concentrations of diuron from 1 to 500 µg·L.