Biofilm development in an open-pore sintered-glass material (SIRAN) was studied using a laboratory-scale, anaerobic fixed-film reactor under thermophilic conditions. The start-up and performance of this reactor; operating on distillery wastewater feed (vinasses), were also studied. Stepped organic loaning during initial reactor start-up reduced the periods of adaptation in the colonisation process and micro-organism attachment, and biofilm formation was accelerated by the surface characteristics of the carrier The results obtained by operating with stepped organic loading (3.81 kg COD/m(3)/day) over a period of 75 days suggest that a stable operation of the process (80% COD removal) and high density of biomass immobilised on the support (89.26 g VSatt/m(3) SIRAN) was achieved. Epifluorescence microscopy demonstrated that, initially, attached growth developed in crevices where biomass was protected from shear forces and, finally, SIRAN was completely covered and biofilm developed on the entire SIRAN particles. The support, under anaerobic thermophilic conditions, due to its properties of low density, high porosity and specific al ea, is suitable for the immobilisation of slow-growing micro-organisms (e.g. anaerobic thermophilic organisms), and is especially adequate as a support for anaerobic fluidised beds for the treatment of high-rate organic lends. (C) 1997 Published by Elsevier Science Ltd.