A pilot study was undertaken to evaluate the efficacy of membrane filtration for the removal of particulate material from two untreated water supplies in Northern California. The permeate produced by the hollow‐fiber ultrafiltration module was assessed by monitoring four water quality parameters: turbidity, suspended solids, direct particle counts, and selected indicator bacteria. The study showed that the ultrafiltration process was very effective in removing particulates from both sources. It appeared that a constant final water quality in relation to particulate material was achieved regardless of the differences in concentrations or numbers of particles in the raw waters.