the performances of two anaerobic sequencing batch reactors (1.2 m3) containing biomass immobilized in inert support and as granular sludge in the treatment of domestic sewage from the campus of s？o carlos-university of s？o paulo were evaluated. the experimental phase lasted seventy days. during this period, the reactors presented quite similar performances in respect to cod and total suspended solids removal, achieving average efficiencies of approximately 60% and 75%, respectively. the analysis using molecular biology techniques on biomass samples taken at 35th and 70th showed differences in the bacterial community in the reactors indicating that the type of biomass immobilization selected the populations differently. a higher similarity was found for the archaea domain probably because these microorganisms utilize specific substrates formed at the end of the anaerobic process.