To account for the transmission of mitochondrial DNA between conspecific species Drosophila pseudoobscura and D. persimilis in sympatry reported by J.R. Powell, a simple model of gene flow and selection in infinite populations is analyzed. The model assumes two alleles at each of two loci, one of which is coded by an autosome and the other by mitochondrial DNA. Viability selection is presumed to be underdominant - i.e., heterozygous inferiority to the homozygotes - at an autosomal locus, and neutral or deleterious at a mitochondrial locus, with the combined action being multiplicative. Extremely strong selection against heterozygotes may prevent the transmission of mitochondrial DNA between two species, but otherwise the transmission can easily occur over species boundaries. The rate of approach to equilibrium is determined by the level of gene flow and is not affected much by selection against an autosomal locus. The divergence of the nuclear genomes of the two species is reexamined. Based on published data on enzyme loci, we conclude that there has been mitochondrial gene flow between these species for a long enough time that several nuclear loci examined could diverge because of accumulation of neutral mutations.