The Bacillus cereus group includes insecticidal bacteria (B. thuringiensis), food-borne pathogens (B. cereus and B. weihenstephanensis) and B. anthracis, the causative agent of anthrax. The precise number of rRNA operons in 12 strains of the B. cereus group was determined. Most of the tested strains possess 13 operons and the tested psychrotolerant strains contain 14 operons, the highest number ever found in bacteria. The separate clustering of the tested psychrotolerant strains was confirmed by partial sequencing of several genes distributed over the chromosomes. Analysis of regions downstream of the 23S rRNA genes in the type strain B. cereus ATCC 14579 indicates that the rRNA operons can be divided into two classes, I and II, consisting respectively of eight and five operons. Class II operons exhibit multiple tRNA genes downstream of the 5S rRNA gene and a putative promoter sequence in the 23S-5S intergenic region, suggesting that 5S rRNA and the downstream tRNA genes can be transcribed independently of the 16S and 23S genes. Similar observations were made in the recently sequenced genome of B. anthracis strain Ames. The existence of these distinct types of rRNA operons suggests an unknown mechanism for regulation of rRNA and tRNA synthesis potentially related to the pool of amino acids available for protein synthesis.