Four serine or threonine codons were introduced into a highly expressed Bacillus subtilis gene. The introduced codons were ones either common in highly expressed B. subtilis genes, or never used in such genes. Strikingly, the level and rate of expression of the modified genes containing either type of extra codons was identical. This suggests that in B. subtilis codon usage patterns may play little or no role in effecting the level of gene expression.