Different from the linear chromosomes of eukaryotes, most chromosomes of bacteria are circular. In this paper, the authors investigated the arrangement of trimers in Bacillus cereus chromosome by recording physical positions of individual trimers along the single-stranded sequence of this chromosome in 5' to 3' direction. The first finding was that numbers of individual trimers in this chromosome were much different. In an attempt to find out how the trimers were arranged along the chromosome, the authors found that every trimer was distributed unevenly and throughout the chromosome. Each trimer was also found to distribute differently on each side of the origin of replication. In addition, frequency of a trimer and that of its reverse complement, such as ATG and CAT, in local regions of the chromosome were always biased. Interestingly, however, distribution of trimers in sense and antisense sequences of the chromosome exhibited an order in their arrangement, getting to conclude that trimer arrangement in the sense and antisense sequences shaped the codon usage of the bacterium at the chromosomal level.