This paper describes the Orange County Water District's experience with silicate fouling of microfiltration (MF) membranes in a demonstration scale MF plant. Symptoms and diagnosis of the problem are included, along with the cleaning method used to remediate the fouling problem. This case study is important due to the unexpected occurrence of this type of fouling on microfiltration membranes and the increasing use of microfiltration in wastewater treatment. Includes 13 references, tables, figures.