Organization and Structure Mitochondrial genomes of higher plants are substantially larger and more variable than those of other organisms. Among the score of plant species investigated, mitochondrial genomes range from 250 to 2500 kb. Within a single family, the cucurbits, 7 to 8 fold size diversity was demonstrated (muskmelon 1600 mdal, cucumber 1000 mdal, zucchini squash 560 mdal and watermelon 220 mdal; Ward et al., Cell 25, 793-803, 1981). These large genomes are composed chiefly of unique sequences (>90%). This remarkable variation is in marked contrast with other mitochondrial genomes (e.g., animals 15-l 8 kb, fungi 18-78 kb, protists 15-47 kb) and the chloroplast genomes of higher plants (120-l 80 kb). Why plant genomes are so large and variable is not understood. It is generally agreed that they may code for a few additional genes not found in other organisms but certainly nowhere near the number needed to account for the extra mitochondrial DNA (mtDNA). “Selfish DNA” would not easily account for the large amounts of nonrepeated, high-complexity DNA encountered in plant mitochondria. It has also been suggested that the extra DNA could serve a “filler” function; however, as yet no mitochondrial functions have been associated with “filler” DNA (Ward et al., op. cit.). The organization of the mitochondrial genome of higher plants is puzzling. Electron microscopy studies characteristically reveal a majority of large linear molecules and lesser amounts of open circular and covalently closed circular molecules (Dale, In Mitochondrial Genes, Slonimski et al., eds., Cold Spring Harbor Laboratory, pp. 471-476, 1982). Generally, the frequency of the circular molecules is low, ranging from about 5% in maize and soybeans to upwards of 40% in tobacco cell cultures. Surprisingly, the circular molecules are most often heterogeneous in both size and relative abundance. For example, in maize at least seven discrete molecular size classes were observed that range from 1.8 kb to as much as 68 kb, and in abundance from 48% to less than 1% of the circles. Molecular heterogeneity appears widespread among plants, since equivalent variation is reported in potatoes, oenothera, Virginia creeper, tobacco, flax and sugar beets. Minicircles and/or minilinears (less than 2.5 kb) are prevalent and are commonly visualized by gel electrophoresis in many species, e.g., maize, tobacco, beans, teosinte. Multimeric series of plant mitochondrial DNAs are reported in several plant species, especially among the smaller size classes. That Minireviews