The transmission of foreign mitochondrial DNA (mtDNA) was investigated in heteroplasmic lines of Drosophila melanogaster constructed by germ-plasm transplantation and maintained at 19 degrees C. When D. melanogaster was used as a germ-plasm donor, donor mtDNA was retained in all four heteroplasmic lines examined. Individual females were found to be heteroplasmic at the 17th and 18th generations. Donor mtDNA derived from D. mauritiana was found to have decreased in all four heteroplasmic lines examined. It could no longer be found after the 16th generation. This difference in the modes of transmission of donor mtDNA in intra- and interspecific combinations of heteroplasmy indicates that there may be certain species-specific functions which propagate and transmit endogenous mtDNA under the nuclear genome of D. melanogaster.