During the course of analysis of the mitochondrial genome of the cellular slime mold, Dictyostelium discoideum, we found a gene (rpl11) for mitochondrial ribosomal protein L11 (RPL11), having 172 amino acid residues. Southern blot analysis revealed that the gene resided in the mitochondrial DNA as a single-copy but not in the nuclear DNA. Prom Northern blot experiments, one major mRNA (about 27 kb) and two minor mRNAs (about 4 and 5 kb) for the gene were detected in the mitochondria. This is the first report showing that the active gene for RPL11 still resides in the mitochondrial genome and has not been transferred to the nuclear genome in D. discoideum.