Nucleotide sequence of a radish chloroplast BafHI-EcoRI fragment (ZL1) derived from B21, which is related to cytoplasmic male sterility and also found in radish 401B retaining line, has been determined. The results show that this fragment is 474bp in length, and is rich in AT base components. The nucleotide sequence of ZL1 has 96.6% homology as compared with the corresponding sequence of tobacco which is located at the positions from 142330 to 142803 in IRA and from 100199 to 99726 in IRB in the tobacco chloroplast genome. It seems that ZL1 contains part of rps7-rps12 operon and encodes 7 of amino acid residues of C-terminal of rps12 as well as 93 residues of N-terminal of rps7. The secondary structure of the partial intron from 1 to 119 in ZL1 was deduced by computer. This three stemloop structure is very similar to that of maize reported by Giese, and it has the CONSENSUS sequence of the Group II intron. The deduced amino acid sequence of S7 in ZL1 shows 95.7, 79.6, 69.9, 40.9, 36.6 and 46.2% and that of S12 shows more than 70% homology compared with the corresponding sequences of tobacco, maize, liverwort, E. gracilis, E. coli and cyanobacterium respectively. The results imply that chloroplast ribosomal proteins may play a role in cytoplasmic male sterility in higher plants.