RNA editing is the alteration of RNA sequences via insertion, deletion and conversion of nucleotides. In flowering plants, specific cytidine residues of RNA transcribed from organellar genomes are converted into uridines. Approximately 35 editing sites are present in the chloroplasts of higher plants; six pentatricopeptide repeat genes involved in RNA editing have been identified in Arabidopsis. However, although approximately 500 editing sites are found in mitochondrial RNAs of flowering plants, only one gene in Arabidopsis has been reported to be involved in such editing. Here, we identified rice mutants that are defective in seven specific RNA editing sites on five mitochondrial transcripts. Their various phenotypes include delayed seed germination, retarded growth, dwarfism and sterility. Mutant seeds from heterozygous plants are opaque. This mutation, named opaque and growth retardation 1 (ogr1), was generated by T-DNA insertion into a gene that encodes a pentatricopeptide repeat protein containing the DYW motif. The OGR1-sGFP fusion protein is localized to mitochondria. Ectopic expression of OGR1 in the mutant complements the altered phenotypes. We conclude that OGR1 is essential for RNA editing in rice mitochondria and is required for normal growth and development.