S . S EL E NS KA -PO BE L L, A. O TT O A N D S . K U TS CH K E. 1998. A highly reliable procedure for fast identification and taxonomical categorization of thiobacilli is presented. The procedure includes RFLP analysis of PCR amplified 16S rDNA, 23S rDNA, and intergenic spacer rDNA between the 16S and the 23S rRNA-genes (amplified ribosomal DNA restriction enzyme analysis -ARDREA), as well as genomic fingerprinting using random primers (RAPD) and repetitive primers (Rep-APD). The taxonomic and discriminatory power of these three analytical approaches is compared. It is demonstrated that the RAPD and Rep-APD methods provide taxonomic results which are in agreement with the classification based on the RFLP analysis of the highly conservative ribosomal RNA (rrn) operons of the strains studied. Moreover, both kinds of genomic PCR fingerprinting, due to the fact that they derive information from different parts of the whole bacterial genome which may possess different degrees of variability, are more informative than ARDREA. By them, a discrimination of closely related Thiobacillus strains is possible. In addition, they are easier to perform and faster than the ARDREA. Using the method described, it was found that one Thiobacillus isolate recovered from uranium waste heaps described in the literature as Thiobacillus ferrooxidans ATCC 33020 is taxonomically neither closely related to the type strain of the species, Thiobacillus ferrooxidans ATCC23270 T , nor to any other strain of this species analysed in the present work.