A section paper selected by the JOURNAL, authored by Lilia McMillan (Active Member, AWWA), sr. electron microscopist, and Roy Stout, microscopist, Wtr. Purific. Lab., City of Chicago, III._____________________________________(Chicago, III.) Iron bacteria have been found in untreated Lake Michigan water for the past ten years. Sphaerotilus, the filamentous bacteria, are the most prevalent. The nonfilamentous bacteria, Caulobacter and Gallionella, found sporadically in the lake, are most common in iron-bearing waters. These organisms are only a minor problem in the Chicago distribution system and are usually found only in dead-end mains; however, they create difficulties in some systems. Various methods for the growth and isolation of iron bacteria are also discussed.