Mitochondrial fusion, recombination, and mobile genetic elements, which are essential for mitochondrial sexuality, are well established in various organisms. The recombination of mitochondrial DNA (mtDNA) depends upon fusion between parental mitochondria, and between their mtDNA-containing areas (mt-nuclei), to allow pairing between the parental mtDNAs. Such mitochondrial fusion followed by recombination may be called "mitochondrial sex." We have identified a novel mitochondrial plasmid named mF. This plasmid is apparently responsible for promoting mitochondrial fusion and crosses over with mtDNA in successive sexual crosses with mF- strains. Only in mF+ strains carrying the mF plasmid did small spherical mitochondria fuse which subsequently underwent fusion between the mt-nuclei that contained the mtDNA derived from individual mitochondria. Several successive mitochondrial divisions followed, accompanied by mt-nuclear divisions. The resulting mitochondria contained recombinant mtDNA with the mF plasmid. Such features remind us also of the bacterial conjugative plasmids such as F plasmid. Therefore, in the final part of this chapter, we discuss the origin of sex and its relationship to the sexuality of mitochondria.