Understanding of introgression pattern in natural populations requires good knowledge of the mechanism and history of gene flow. Postmating and prezygotic isolation between Bradybaena pellucida (BP) and B. similaris (BS) is not reciprocal. In general, only BP produces F₁ hybrids while BS does not reproduce because BP ceases copulation without insemination of BS. Thus, mtDNA would leak from BP to BS populations more easily than in the opposite direction. Against this prediction, the heterospecific haplotype has been found significantly more frequently in BP than BS in the Boso peninsula, where BP has been spread recently. This introgression pattern may have resulted from past introgression but not from recurrent introgressions; BP individuals carrying the BS haplotype may have just founded in the current populations examined. In contrast, no introgression has so far been detected in either BP or BS in western Japan, where BP is known to have long been indigenous. Lineage sorting expected in populations in a long term explains the presence and absence of heterospecific mtDNA haplotypes in Boso peninsula and western Japan, respectively.