Abstract Following the operation of three laboratory-scale slow sand filters, containing three different sand grain sizes, over a period of 2 years samples of the sand were taken from each of the filters from nine different depths below the surface. These samples were initially shaken in a small volume of water to detach any accumulated solids. The suspended solids content and the turbidity of the carriage water was then determined. In addition all sand samples were examined for particulate organic carbon (POC) and for the bacterial content by means of a standard plate count, as well as being observed both by a scanning electron microscope (SEM) and by means of epifluorescence microscopy (EM). Of these techniques it was the determination of the bacteria shaken free from the sand grains which gave the most reproducible results. No worthwhile results were obtained as a result of EM but the SEM proved to be a valuable support for the quantitative techniques employed. The presence of organic solids was demonstrated throughout the depths of all three filters while generally there was a noticeable reduction in the number of bacteria below the depth of 400 mm.