A great number of aerobic endospore-forming Gram-positive Bacillus species have been isolated on a number of occasions from a variety of terrestrial and deep-sea environments, including the Mariana Trench which has a depth of 10,897 m. Some of these Bacullus species are known to have various capabilities for adapting to extreme environments. In fact, Bacillus-related species can grow in a wide range of environments -- at pH 2-12, at temperatures between 5 and 78°C, in salinity from 0 to 30% NaCl, and under pressures from 0.1 Mpa to at least 30 Mpa. The author is now exploring how these adaptive capabilities, as reflected in their genomes, were acquired and what intrinsic genomic structures are present in Bacillus-related species that have allowed them to adapt to such a wide range of environments. To answer these questions, the author initiated a genome sequencing project in early 1998 and have to present determined the entire genomic sequences of three extremophilic bacilli: alkaliphilic Bacillus halodurans, extremely halotolerant and alkaliphilic Oceanobacillus iheyensis, and thermophilic Geobacillus kaustophilus. The author provides the first comparative analysis of the extremophilic bacillar genomes with those of three other phylogenetically related mesophilic and neutrophilic bacilli, B. subtilis, B. anthracis and B. cereus, in order to highlight the commonality and diversity of the bacillar genome.