We have isolated cDNA clones specific for Arabidopsis thaliana cytosolic ribosomal protein S11 and plastid ribosomal protein CS17, both of which are encoded in the nuclear genome, through the use of the corresponding soybean and pea cDNAs as probes, respectively. The nucleotide sequences of all four cDNAs were determined. The amino acid sequences derived from these cDNA sequences show that the soybean and A. thaliana S11 cDNAs encode proteins that are homologous to rat ribosomal protein S11 and that the pea and A. thaliana CS17 cDNAs encode proteins that are homologous to Escherichia coli ribosomal protein S17. The plant S11 cytosolic ribosomal proteins also show significant sequence similarity to both E. coli ribosomal protein S17 and plastid CS17 indicating that these are all related proteins. Comparison of A. thaliana CS17 with A. thaliana S11 and with E. coli S17 suggests that CS17 is more related to S17 than it is to S11. These results support the idea that the gene encoding CS17 was derived from a prokaryotic endosymbiont and not from a duplication of the eukaryotic S11 gene.