One of the plant genomes, mitochondrial (mt) DNA, of potato has not been as extensively investigated as the other 2 genomes, nuclear and chlroplast DNAs. Restriction fragment length polymorphisms of mt DNA of potato were studied to analyze the mt DNA variation of potato. The mt DNA was isolated from leaves of 21 Japanese potato cultivars (Solanum tuberosum ssp. tuberosum) and a stain of S. tuberosum ssp. andigena. The mt DNAs were analyzed by restriction endonuclease analysis using 8 endonucleases (BamHI, EcoRI, HindIII, PstI, PvuII, Sall, SmaI and XhoI) and Southern hybridization analysis using the endonucleases and 3 rice mt DNAs (coxI atpA and atp 6 genes) as probes. The HindIII, SmaI and XhoI digests gave 2 different patterns, and PvuII and SalI gave 3. Based on the fragment patterns, the mt genome of the potatoes was classified into 4 types. The combination of HindIII/atpA revealed 3 hybrid band patterns, and 5 combinations of BamHI/atpA, EcoRI/atpA, EcoRI/coxI, SalI/atpA and XhoI/coxI revealed 2. The mt genome was classified into 3 types based on the hybrid band patterns. Based on the data of both analyses, the mt genome of the potatoes was divided into 5 groups.