We have characterised a cDNA clone and a nuclear gene encoding the chloroplast 30 s ribosomal protein S13 from Arabidopsis thaliana. The identification is based on the high similarity of the predicted amino-acid sequence with eubacterial S13 protein sequences, and immunodetection of a 14.5-kDa chloroplast ribosomal polypeptide using antibodies raised against the polypeptide produced from part of the cDNA expressed in bacteria. The predicted amino-acid sequence contains an N-terminal extension which has several features characteristic of chloroplast transit peptides. Experiments suggest there is a single copy of this gene in A. thaliana and multiple copies in Brassica species. The origin of the mitochondrial S13 polypeptide in crucifers is also discussed.