Abstract: In this talk, I will show that mitochondrial genomes are surprisingly active in horizontal gene transfer (HGT). One, enormous plant mitochondrial genome is far richer in foreign genes than even the most HGT-rich bacterial genome. Among other things, this genome has incorporated entire mitochondrial genomes from distantly related donors. Analysis of this and other mitochondrial genomes of plants and fungi leads us to propose a molecular/cellular model explaining why mitochondrial genomes are so active in HGT. Reasons why HGT is especially prominent in mitochondrial genomes of plants will also be presented, as will mechanisms by which genes move from one plant to another. Many horizontal transfers in plants and fungi give rise to chimeric mitochondrial genes in which pieces of foreign genes are pasted into native copies. Often these occur in very short patches, leading us to suggest that many, apparently de novo mitochondrial mutations may in fact be the result of unrecognized microchimeric HGT.