FOR the past two years I have been engaged in the investigation of the iron-water of Scotland, England, and Wales, and altogether have examined over a hundred samples. This publication is a preliminary notice of some new species of iron-bacteria which I have found in the course of this investigation. At present the number of known forms belonging to this class is six. In this paper I wish to outline the main characteristics of five new species, reserving for later papers the detailed accounts of their life-histories.