DNA WAS ISOLATED FROM A MITOCHONDRIAL FRACTION OF EACH OF THE FOLLOWING PLANT MATERIALS: Mung bean (Phaseolus aureus) etiolated hypocotyl; turnip (Brassica rapa) root; sweet potato (Ipomoea batatas) root; and onion (Allium cepa) bulb. It was found that all of these mitochondrial fractions contained DNA, the densities of which were identical (rho=1.706 g.cm(-3)). An additional DNA (rho=1.695) band found in the mitochondrial fraction of Brassica rapa, was identical to DNA separately isolated from the chloroplast-rich fraction. The origin of the second DNA from Allium mitochondrial fraction was not identified.Contrary to the identity of the mitochondrial DNA, DNA from nuclear fractions differed not only with each other but from the corresponding mitochondrial DNA.DNA from Phaseolus and Brassica mitochondria showed the hyperchromicity characteristic of double stranded, native DNA upon heating; Tm's in 0.0195 Na(+) were the same; 72.0 degrees . The amount of DNA within the mitochondrion of Phaseolus was estimated to be 5.0 x 10(-10) mug; this estimate was made by isolating the mitochondrial DNA concomitantly with the known amount of added (15)N(2)H B. subtilis DNA (rho=1.740). Approximately the same amount of DNA was present in the mitochondrion of Brassica or Ipomoea.