Horizontal gene transfer has been recognized as a universal event throughout bacterial evolution. The availability of both complete genome sequences of Bacillus cereus and Bacillus anthracis provides the possibility to perform comparative analysis based on their genomes. By using a windowless method to display the distribution of the genomic GC content of B. cereus and B. anthracis, we have found three genomic islands in the genome of B. cereus, i.e., BCGI-1, BCGI-2 and BCGI-3, respectively, which are absent in the genome of B. anthracis. All the genomic islands have abrupt changes in GC content compared with that of surrounding regions. BCGI-1 has many conserved features of genomic islands, e.g., a Val-tRNA gene is utilized as the integration site, and a site specific recombinase gene is located at the 3' end. BCGI-2 has a large percentage of phage-protein, suggesting a phage related recombination is involved. BCGI-3 contains a ferric anguibactin transport system, which is likely to be involved in the iron transport that enables the bacterium to overcome the iron limitation in the host. In addition, BCGI-3 also contains a cluster of genes related to lantibiotics, which may play a role during the evolution of the genome. Furthermore, the integrations of the genomic islands, BCGI-1 and BCGI-3, result in deletions of DNA sequence fragments, therefore, such integrations lead to both gene gain and gene loss simultaneously.