Blackish brown particles of manganese oxide usually derived from the microorganism Metallogenium were observed for the first time in November, 2002 at the center of the north basin of Lake Biwa off Imazu (water depth, 90 m). However, the taxonomic position and biochemistry of Metallogenium remain poorly understood since a stable culture of this microorganism has seldom been successfully established. Using Lake Biwa water samples, the authors have prepared several stable cultures that continually produce Metallogeium -like particles of manganese oxide. In some cultures, suck particles were produced in association with fungi, though only bacteria were found in the other cultures. Since fungi are generally rare in lake waters, the production of Metallogenium by bacteria is presumably the major agent for the oxidative precipitation of Mn2+ in those waters. Metallogenium appeared within 2 weeks after inoculation in the coexisting fungus cultures. However, in the cultures with bacteria only, it took 4-6 weeks before Metallogenium could be detected in the medium. In this paper, we mainly reported morphological variations of Metallogenium observed by light and scanning electron microscopy during the development of those cultures in which only bacteria were found.