A low density support material was evaluated to determine its effect on biofilm growth dynamics when treating synthetic wastewater in a three-phase fluidized-bed reactor. After two 40-day experimental runs, the results showed significant microbial activity from the very beginning of the study. The highest biofilm growth was observed during the first 10 days of operation. Biofilm kept gradually growing after the 11th day and reached a steady-state at day 21, defined by a consistent biomass attached to the particles. As an indicator of biofilm detachment effluent suspended biomass decreased as biofilm attachment increased, tending also to stabilize around day 21. During the first 10 days, chemical oxygen demand removal averaged 100 mg l(-1) representing 48% of the influent chemical oxygen demand concentration. The highest chemical oxygen demand removal efficiency (78%) was achieved between days 23 and 29, when influent chemical oxygen demand was 280 mg l(-1). The support material evaluated proved to be effective at allowing attachment of microorganisms. Due to its low density, bed fluidization was achieved under recirculation rates lower than those required to fluidize beds of higher density. The physical properties of the proposed support material allowed an appropriate equilibrium to be achieved between biomass attachment and detachment, eliminating the need for bed backwashing.