The Gilaki and Mazandarani occupy the South Caspian region of Iran and speak languages belonging to the North-Western branch of Iranian languages . It has been suggested that their ancestors came from the Caucasus region, perhaps displacing an earlier group in the South Caspian . Linguistic evidence supports this scenario, in that the Gilaki and Mazandarani languages (but not other Iranian languages) share certain typological features with Caucasian languages . We analyzed patterns of mtDNA and Y chromosome variation in the Gilaki and Mazandarani. Based on mtDNA HV1 sequences, the Gilaki and Mazandarani most closely resemble their geographic and linguistic neighbors, namely other Iranian groups. However, their Y chromosome types most closely resemble those found in groups from the South Caucasus. A scenario that explains these differences is a south Caucasian origin for the ancestors of the Gilaki and Mazandarani, followed by introgression of women (but not men) from local Iranian groups, possibly because of patrilocality. Given that both mtDNA and language are maternally transmitted, the incorporation of local Iranian women would have resulted in the concomitant replacement of the ancestral Caucasian language and mtDNA types of the Gilaki and Mazandarani with their current Iranian language and mtDNA types. Concomitant replacement of language and mtDNA may be a more general phenomenon than previously recognized.