We have used a variety of methods to localize the genes for ribosomal proteins S7 and S12 on Spirodela chloroplast DNA. Heterologous hybridization with a rps12 gene specific probe from Euglena has revealed the presence of rps12 homologous sequences within the inverted repeat of Spirodela chloroplast DNA on the fragment BamHI-V. In the partial nucleotide sequence of this fragment, two regions of amino acid sequence homology to Euglena S12 can be identified, separated from each other by a 542 bp intron with conserved boundary sequences. As was found for Nicotiana S12, the Spirodela S12 coding regions are for 85 amino acids homologous (79%) to E. coli S12 (starting from residue 38 to the C-terminus). Likewise, we are unable to identify the 37 5' terminal codons of rps12 in Spirodela. The functionality of the Spirodela rps12 sequence is discussed. The rps7 gene is located adjacent to rps12. Chloroplast ribosomal protein C-S11 (homologous to S7) has been detected by immunoprecipitation with both a polyspecific anti 30S serum and an anti C-S11 serum, among the in vitro translation products of mRNAs selected by Spirodela chloroplast DNA fragments BamHI-V and BamHI-P. Since in a DNA dependent E. coli cell free system, only BamHI-V appears to be capable of synthesis of C-S11, it is concluded that rps7 is located entirely within BamHI-V and is transcribed into a mRNA which extends into BamHI-P. As determined by Northern hybridization experiments, rps7 is cotranscribed with rps12; a stable transcript of approx. 1100 b is detected in total cellular Spirodela RNA with either rps12 and rps7 gene specific probes. The rps12 probe additionally detects an approx. 600 b transcript, which presumably corresponds to the excised rps12 intron RNA. Finally we have examined the expression of both rps7 and rps12 during light induced chloroplast development by Northern blotting and by immunoblotting. It is shown, that the steady-state levels of neither chloroplast ribosomal protein transcripts, nor those of the chloroplast ribosomal proteins itself, change significantly during the greening process.