This study assessed the effects of substratum roughness, using two grades of silicon carbide sandpaper coupons, and river flow rate on the colonisation dynamics of.bacteria, heterotrophic flagellates and algae over a 16 week incubation period in the River Conder, UK. In the short-term i.e. up to 2 weeks, a combination of substratum roughness and high river flow significantly increased bacterial biomass on the coupons, but did not affect any of the numbers cm(-2) of other organisms. In the long-term i.e. up to 16 weeks, neither parameter appeared to affect the development and maintenance of the mature biofilms. Evidence of a diverse protozoan community within the biofilms was recorded, thereby highlighting the need to consider the effect of these primary consumers on biofilm dynamics in future studies.