The sequences of the rps4 and rps10 genes encoding the Dictyostelium discoideum homologues of the basic ribosomal proteins S4 and S10 were determined from cDNA and genomic DNA clones. They are expressed respectively as 266 and 153 amino-acid-long proteins. In both cases, the N-terminal methionine is cleaved in the mature proteins. S4 contains two putative nuclear targeting signals and displays a strong overall identity (around 60%) to eukaryotic S4 homologues. The rps10 gene harbours a 314-bp intron located close to its 5'-coding end. The overall identity between D. discoideum S10 and eukaryotic homologues is around 38% and rises to 53% in the N-terminal domain. Southern blots suggest that both S4 and S10 are encoded by single genes that are regulated during development. The corresponding mRNAs decrease sharply after 8 h of differentiation.