The deposition and removal of submicron bacteria (SW8) has been examined by the direct observation through the membrane (DOTM) technique. The original DOTM was modified to incorporate fluorescence microscopy to better visualise the submicron material. The flux at which deposition commenced, the so-called critical flux,, was visually identified before the transmembrane pressure indicated cake formation. After supercritical operation for about 15 min the flux was reduced to subcritical and then zero; slow cake removal was observed as distinct cylindrical rolling floc (about 50 Km diameter) and aggregates. The extent of cake removal varied from about 90 to <5% depending on the ionic environment with low ions resulting in better removal. Cake formed over longer periods (upto 60 min) showed negligible removal. The critical fluxes of SW8 measured with DOTM increased with crossflow, but exhibited higher values than expected from their primary particle size. (C) 2003 Elsevier Science B.V. All rights reserved.