Mitochondrial DNA (mtDNA) molecules from species of the genus Drosophila contain a region exceptionally rich in adenine + thymine (A+T). Using agarose gel electrophoresis and electron microscopy, we determined that in the mtDNA molecules of D. melanogaster, D. simulans, D. mauritiana, D. yakuba, D. takahashii, and D. virilis, the A+T-rich regions, which are 5.1, 4.8, 4.6, 1.1, 2.2, and 1.0 kilo...