Northern analyses and reverse transcription-polymerase chain reaction (RT-PCR) experiments were performed on total RNA of Dictyostelium discoideum. The mitochondrial genes encoding the small subunit ribosomal RNA (SSU), cytochrome b (CYTB) and subunit 3 of the NADH dehydrogenase (ND3) were found to be co-transcribed. Further post-transcriptional processing resulted in a dicistronic transcript for CYTB and ND3, and a monocistronic SSU transcript. Markedly higher steady state transcript levels were detected for the mature SSU ribosomal RNA. A comparison of the SSU cDNA sequence with the mitochondrial DNA sequence of the SSU gene revealed C-to-U substitutional editing of the SSU ribosomal RNA at a single site, as a consequence of which the cDNA contained a PvuII site not present in the genomic DNA. The editing was shown to be highly efficient and to occur in the primary transcript before the release of the mature mRNA, rRNA and tRNAs. It is suggested that the editing may be required for normal pseudoknot formation in the 530 loop of the RNA and thus is important for efficient, accurate translation in the mitochondria.