In this work, the variability of spo0A gene in the genus Geobacillus and applicability of this gene for the taxonomy within this genus were evaluated. The protein Spo0A is the master regulator of the endospore-forming process in the all endospore-forming bacteria. Geobacillus genus-specific primers GEOSPO were designed based on the sequences of Geobacillus spo0A gene available through the public databases. Inter and intraspecific variability of Geobacillus spo0A gene was determined after sequencing of the GEOSPO-PCR products. Geobacillus spo0A sequence analysis showed that three species--Geobacillus thermodenitrificans, G. stearothermophilus, and G. jurassicus--could be easily identified. Similarity between the sequences of these species and the other species were in the range of 83.3%-92.0%. In contrast, intraspecific similarity of G. thermodenitrificans and G. stearothermophilus was high--above 99.0%. Similarity of spo0A sequences of G. subterraneus-G. uzenensis species cluster also matched this interval. Intercluster similarity between G. lituanicus-G. thermoleovorans-G. kaustophilus-G. vulcani and G. thermocatenulatus-G. gargensis-G. caldoxylosilyticus-G. toebii-G. thermoglucosidasius species clusters, as well as interspecific similarity within these two clusters was in the range of the intraspecific similarity determined for G. thermodenitrificans and G. stearothermophilus. It was also determined that spo0A cannot be used as the phylogenetic marker for the genus Geobacillus.