Cadmium is a non-essential trace metal, with elevated concentrations in many aquatic systems. The bioavailability of Cd as well as of other non- essential and essential metals depends not only on their concentrations but is also strongly dependent upon chemical speciation. Periphyton is the predominant primary producer in surface waters and therefore of ecological importance. The goals of this work were to gain detailed insight into the kinetics of Cd accumulation by periphyton, and to investigate which Cd species control bioaccumulation in natural freshwaters. Experiments were carried out in artificial channels supplied with natural freshwater, with controlled Cd exposure concentrations and speciation. A field study was performed to investigate the relationships under natural conditions. The results show that periphyton sensitively responds to changes of environmentally relevant Cd concentrations in water and accumulates Cd at low ambient concentrations. Labile metal complexes with organic ligands might control bioaccumulation in periphyton in natural freshwaters. Thus, dynamic metal species should be considered in models predicting bioavailability.