Supercoiled mtDNAs were isolated from tissue culture cells of tobacco, bean, and corn, and the smallest size classes were used to study the relationships among the different size classes of each species through restriction digests and hybridizations. Three of the smallest tobacco mtDNAs [10.1, 20.2, and 28.8 kilobases (kb)], the two smallest bean mtDNAs (1.9 and 3.8 kb), and the two smallest corn mtDNAs (1.5 and 1.8 kb) were extracted from the gels and nick translated. The 10.1-kb tobacco mtDNA hybridizes to all the other tobacco mtDNA size classes and a large percentage of the tobacco mtRNAs. Restriction digests indicate that the 20.2-kb size class is a dimer of the 10.1-kb size class. The 1.9-kb bean mtDNA hybridizes to all but three of the bean mtDNA size classes and hybridizes to two mtRNAs. Restriction digests indicate that the 3.8-kb size class is a dimer of the 1.9-kb size class. The 1.5- and 1.8-kb corn mtDNAs, which do not have any Hha I restriction fragments in common, both hybridize to many of the same size classes of the corn mtDNA profile and, in addition, each hybridizes to a few size classes not recognized by the other. The 1.5- and 1.8-kb size classes both hybridize to two RNAs, one of which they appear to have in common. However, with both the 1.9-kb bean mtDNA and the two corn mtDNAs, the molecular sizes of the two RNAs exceed those of the respective DNAs. The possible role and origin of the many size classes are discussed.