The effect of the support characteristics on die substrate degradation kinetics ann on the adherence ann gr growth of biofilms is a subject of considerable importance in wastewater treatment. Many works deal with the kinetics, but less, work has been reported concerning the surface differences. III this paper; the results of an experimental programme studying the biofilm growth in rotating biological reactors (RBC) with different solid material in each biodisk, are presented. Results from two phases have been obtained; the first phase of colonization, of about 1-2 days, was followed by a growth phase with a specific growth rate constant between 1.1-1.3 d(-1). The biofilm dry matter (DM) ranged between 7 mg DM/cm(2) for the glass pal-tide ann diatomite earth support materials, and 9 mg DM/cm(2) for the sand disk and 10 mg DM/cm(2) for the activated carbon disks. All these results indicated that car bon was the best of the tested support materials for biofilm formation. No dependence was observed between position of the biodisk nl the RBC and the organic layer formation or the initial biofilm development.