A 23S ribosomal (rRNA) gene from Bacillus stearothermophilus has been cloned in pBR322, and its nucleotide sequence determined. The corresponding mature 23S rRNA is predicted to contain 2928 nucleotides. We compare the primary and secondary structures of 23S rRNA from Escherichia coli and B. stearothermophilus, and discuss their potential contributions to thermal stability.