In this paper we present a simple in expensive substrate for studying biofilms. Bioballs are robust plastic spheres with high surface area to volume ratios (ca 130:1) used in home aquarium filters. Their properties make them ideal substrates for use in studies of aquatic biofilms. As examples, in this paper we describe the growth and enzyme activities of biofilms grown on these substrates and explore the interaction between DOC concentration and biofilm biomass. Biofilm growth and activity on the bioballs was very reproducible. For example maximum biofilm growth following approximately 3 months inundation was estimated to be 40 (+/- 3.6) mg protein and 120 (+/- 11) microg chlorophyll a per bioball.