The occurrence, in pipelines carrying freshwater, of a deposit containing 20-40% manganese, has been investigated in Tasmania. Heavy deposits form rapidly in pipelines carrying Lake King William water while those carrying Great Lake water remain free from deposits over long periods. An apparatus was designed for studying the problem on a laboratory scale and it was proved experimentally that the manganese deposits are initiated by microorganisms. The lack of deposits in pipelines carrying Great Lake water results from lack of available manganese in the water, since addition of soluble manganese causes a deposit to form.