The growth of seven Thiobacillus ferrooxidans strains was trested on various solid media prepared according to previously published procedures in comparison with a series of new formulations (TSMs, Thiobacillus solid media) set up in our laboratory. These media differed in the type of gelling agent and in the concentration of phosphate and ferrous ions. It was determined that the established formulations brought qualitatively and quantitatively unsatisfactory results whereas, among the new media, TSM 1 produced quantitative yields of T. ferrooxidans colonies. The TSM 1 supports the growth of T. ferrooxidans strains and can be recommended for the estimation of cell numbers in liquid cultures and for the isolation of single clones. There was also a noticeable similarity in the morphology of colonies formed by different Ts ferrooxidans strains.The chemoautotrophic bacterium Thiobacillus ferrooxidans is able to obtain energy from the oxidation of ferrous iron and/or inorganic sulfur compounds. An important feature of this extremophile is its ability to grow and reproduce readily at the lower extremes of the pH scale (1.5-2.0) (3).Studies of the biochemistry and physiology of this microorganism have been stimulated by the ever increasing development of the bioleaching processes that are used industrially to recover metals from ores and to desulphurize coal (2,11). In spite of this interest in T, ferrooxidans, and its genetic improvement for biomining operations (15) much work has still to be done to select efficient strains for making the leaching systems commercially feasible. The lack of studies of the genetic manipulation of this bacterium is mainly concerned with the difficulty of applying * Address reprint requests to: Dr .