Two different uranium mine waste heaps near Ronneburg, Thuringia, Germany, which contain the remains of the activity of the former uranium-mining Soviet-East German company Wismut AG, were analyzed for the occurrence of lithotrophic and chemoorganotropic leach bacteria. A total of 162 ore samples were taken up to a depth of 5 m. Cell counts of ferrous iron-, sulfur-, sulfur compd.-, ammonia-, and nitrite-oxidizing bacteria were detd. quant. by the most-probable-no. technique. Sulfate-, nitrate-, ferric iron-, and manganese-reducing bacteria were also detected. In addn., the metabolic activity of sulfur- and iron-oxidizing bacteria was measured by microcalorimetry. Generally, all microorganisms mentioned above were detectable in the heaps. Aerobic and anaerobic microorganisms thrived up to a depth of 1.5 to 2 m. Up to 99% of Thiobacillus ferrooxidans cells, the dominant leaching bacteria, occurred to this depth. Their nos. correlated with the microbial activity measurements. Samples below 1.5 to 2 m exhibited reduced oxygen concns. and reduced cell counts for all microorganisms.