The mitochondrial genome of mitochondrial type (mitotype) SW3 of carrot (Daucus carota L.) encodes intact reading frames for a RNA polymerase (Rpo) and a DNA polymerase (Dpo) similar to those encoded by linear mitochondrial plasmids from plants. A BLAST search of translated nucleotide sequences in GenBank revealed previously unreported plasmid-like Rpo or Dpo sequence fragments in many plant mitochondrial DNAs. Phylogenetic analyses of the relationships between mitochondrial (mt)DNA-encoded and plasmid-encoded Rpos and Dpos from plants suggest that the mitochondrial sequences were derived from integrated plant plasmid sequences. A linear mitochondrial plasmid was detected in a different mitotype (FG21) of carrot by Southern hybridization of the Rpo and Dpo to undigested mtDNAs. Transcripts of the mtDNA-encoded Rpo and Dpo in mitotype SW3 were detected by RT-PCR.