The degree of the genetic variations among Clostridium thermocellum ATCC 27405 and the wild type strains was investigated by the mehtod of GC ratio, DNA-DNA hybridization and RFLP (Restriction Fragment Length Polymorphism) patterns by ribosomal RNA and M13 probe. GC ratio and KNA homology values of th three isolates were approximately equal to those of ATCC type strain. The RFLP patterns by the rRNA and M13 probe showed some differences among C. thermocellum ATCC 27405, wild type strains and Clostridium thermohydrosulfuricum ATCC 33223, indicating that the two probes can be useful in subspecies- and apecies-identification.