Community structures of submerged microbial slime streamers (SMSS) in sulfide-containing hot springs at 72 to 80 degrees C at Nakabusa and Yumata, Japan, were investigated by molecular analysis based on the 16S rRNA gene. The SMSS were classified into two consortia; consortium I occurred at lower levels of sulfide in the hot springs (less than 0.1 mM), and consortium II dominated when the sulfide levels were higher (more than 0.1 mM). The dominant cell morphotypes in consortium I were filamentous and small rod-shaped cells. The filamentous cells hybridized with fluorescent oligonucleotide probes for the domain Bacteria, the domain Archaea, and the family Aquificaceae: Our analysis of the denaturing gradient gel electrophoresis (DGGE) bands by using reverse transcription (RT)-PCR amplification with two primer sets (Eub341-F with the GC clamp and Univ907R for the Bacteria and Eub341-F with the GC clamp and Arch915R) indicated that dominant bands were phylogenetically related to microbes in the genus Aquifex: On the other hand, consortium II was dominated by long, small, rod-shaped cells, which hybridized with the oligonucleotide probe S-*-Tdes-0830-a-A-20 developed in this study for the majority of as-yet-uncultivated microbes in the class Thermodesulfobacteria: The dominant DGGE band obtained by PCR and RT-PCR was affiliated with the genus Sulfurihydrogenibium: Moreover, our analysis of dissimilatory sulfite reductase (DSR) gene sequences retrieved from both consortia revealed a high frequency of DSR genes corresponding to the DSR of Thermodesulfobacteria-like microorganisms. Using both sulfide monitoring and (35)SO(4)(2-) tracer experiments, we observed microbial sulfide production and consumption by SMSS, suggesting that there is in situ sulfide production by as-yet-uncultivated Thermodesulfobacteria-like microbes and there is in situ sulfide consumption by Sulfurihydrogenibium-like microbes within the SMSS in the Nakabusa and Yumata hot springs.