The growth of Gallionella ferruginea in a mineral salts solution with carbon dioxide and iron sulphide was studied by acridine orange stained direct count and most probable number techniques. G. ferruginea grew to 2 .times. 106 cells ml-1 with a generation time of 8.cntdot.3 h under aerobic gradient conditions. The optimum temperature for growth was 20.degree.C. No growth was obtained under anaerobic conditions, or without carbon dioxide. A method was developed for measuring the length of the stalks formed by G. ferruginea. When growing exponentially, the bacterium was free-living, without stalks, and motile with one polar flagellum. A net production of stalk per cell began when the cell number exceeded 6 .times. 105 ml-1 if the pH exceeded 6. This occurred when growth entered the stationary phase. The stalk length increased from 3 .times. 103 .mu.m ml-1 (detection limit) to 1.cntdot.8 .times. 108 .mu.m ml-1, during a 400 h growth experiment. There was no stalk formtion at growth conditions where ferrous iron was stable, suggesting that stalk formation may be a protection mechanism against an increasing reducing capacity of ferrous iron as it becomes unstable in an environment that becomes oxidized. The results indicate that favourable growth conditions for G. ferruginea may be those present in reduced ground waters, rather than those in ditches, drainage tubes, wells, etc., where stalk-forming G. ferruginea can usually be found.