A new species and genus of colourless sulphur bacteria, for which the name THIODENDRON mucosum is proposed, is described from two habitats, in Florida. Its morphology and physiology relate it to the Beggiatoaceae, but it is branching, nonfilamentous and non-motile so it should not be included in that family and a new family should be created for it Some characteristics of its enrivonment are discussed and also the organisms common to such an environment. It is probable that the particular organisms are closely connected to the metabolism of sulphur, while their variety and biomass attribute an important role to sulphur in nature, especially in environments containing little or no oxygen.