The recent applications of reverse osmosis and nanofiltration to surface water have revived interest in instruments that measure the fouling potential of the pretreated water to a membrane train. The objectives of this paper are to describe and present the evaluation results of a new on-line instrument to measure the Silt Density Index (SDI). The SDI is an industry standard test based on the measurement of a water sample flow through a 0.45 um filter pad. Anjou Recherche, a subsidiary of Compagnie Generale des Eaux, has developed the SILTIMETER, an automated on-line SDI apparatus based on the use of standard 25 mm diameter filter cartridges. The apparatus has two channels (two water sources) and can make measurements at a variable frequency up to 48 per day (15 minutes for the measurement, 15 minutes for rinsing and filter change). The SILTIMETER has been evaluated for one year on a sand filtered surface water in Mery-sur-Oise, France.