Size and shape of mitochondrial DNA molecules of Schizosaccharomyces pombe were analyzed by electron microscopy. Besides numerous linear molecules, circular molecules ranging from 0.83 micron to 12.81 micron were found. Depending on the method of preparation, both closed and open circular molecules were found. Most of the circular molecules could be assigned to five major size classes of 0.83 +/- 0.05 micron, 1.7 +/- 0.05 micron, 4.74 +/- 0.04 micron, 5.74 +/- 0.04 micron, and 8.32 +/- 0.07 micron. Possible explanations for the different size classes of mitochondrial DNA molecules are discussed.