A northern California treatment plant installed microfiltration to satisfy design constraints and comply with SWTR requirements. With the enactment of the federal Surface Water Treatment Rule (SWTR) and a state SWTR, San Jose Water Company faced a decision to either abandon or upgrade a small diatomaceousearth (DE) filtration plant. Several environmental, design, operational, and regulatory constraints influenced the decision to replace the existing DE filtration system with microfiltration (MF). At 5 mgd this is currently the largest potable water MF plant in the United States. It has been operating successfully since February 1994. Results to date demonstrate that the plant is producing finished water that exceeds SWTR requirements. This article describes elements of the design, construction, operation, and performance of the new plant as well as factors leading to the selection of MF over other treatment options.