As part of a water source development project in Lake Havasu City, Arizona, a new 26 mgd water treatment plant has recently been constructed. The well water contains high levels of manganese, 1.0 to 1.2 mg/L, that in the past created problems such as black water staining and precipitation of manganese dioxide on the interior of the distribution system pipes. An innovative treatment process using biological filtration to remove the manganese was tested and selected for the treatment plant. This paper describes the biological manganese removal process, reviews the results from the pilot study, summarizes the full-scale design and presents the initial performance from North America's largest biological manganese removal plant. Includes 6 references, figures.