A microcosm study was undertaken to examine the effects of dissolved cadmium at various concentrations (0, 10, and 100 μg · L−1) on biofilm accumulation and diatom assemblages. A natural biofilm sampled from the Riou-Mort River (Southwest France) was inoculated into three experimental systems, where biofilm settled on glass slides. Samples collected after 1, 2, 4, and 6 weeks of colonization were analyzed for metal accumulation (total metal content and intracellular metal content in the biofilm), biomass (as measured through dry weight and ash-free dry matter), and quantitative as well as qualitative analysis of diatom assemblages. There was a positive correlation between cadmium accumulation and dissolved cadmium concentrations and duration of exposure: a linear relationship was found between concentration factors (CFs) of growing biofilms and time (CFs/day = 0.25 and 0.38 under contaminations of 10 and 100 μgCd · L−1, respectively). Biofilm settlement, more than photosynthetic activity, was affected by high cadmium concentrations: we observed for all stages of settlement a drastic and significant (p < 0.05) reduction in biofilm biomass and in diatom densities in the highest cadmium contamination, compared to control and low cadmium concentration units.