The formation of biofilms on heat exchange surfaces was studied using water contaminated with Pseudomonas fluorescens. The rate of deposition and the maximum amount of deposit decrease as the fluid velocity increases. The processes occurring at the interface and in the biofilm appear to govern the build-up of the deposit. together with the removal (shear stress) mechanism. Values of the attachment and biological growth rates were calculated from the changes observed in the biofilm thermal resistances after suppressing the addition of nutrients to the flowing water. In both cases there is a decrease for higher velocities.