The aim of this work was to investigate the heavy metal accumulation by natural biofilms living in the catchment area of the Tisza river in Hungary, as well as in biofilms cultivated in vitro. Laboratory tests have demonstrated that metals can be adsorbed on biofilms, depending on their concentration and on the availability of free sorptive places. Biofilms were cultivated in vitro in natural freshwater from the Saale river, Germany. After reaching the plateau phase, Cu was added to reach a concentration of 100 µg/L. An increase of its mass fraction in the biofilm was observed, which caused the decrease of the concentration in the water phase. Unfortunately, the reactor wall was also found to act as adsorbent for Cu. More detailed results of our in vitro experiments will be published in a forthcoming paper. Naturally grown biofilm samples from exposed as well as background places at the Hungarian rivers Szamos and Tisza were collected in 2000 and 2002 after the cyanide spill, and analysed using total reflection X-ray fluorescence analysis (TXRF). Metal mass fraction differences as high as two orders of magnitude were found between polluted and unpolluted (background) sampling points. Extremely high concentration values, e.g. 5600 µg/g Zn in biofilm, were found at highly polluted sampling points. This means an enrichment factor of ca. 10,000 compared to the water phase.