This paper is a first attempt at understanding the impact of Islam on families in eighth-century rural Ṭukhāristan (modern-day northern Afghanistan), at the periphery of the late Umayyad and early ʿAbbāsid caliphate. Tukhāristan lay in the ancient region of Bactria, which became the land and city of Balkh after the Islamic conquests of the early seven hundreds ad. My analysis is based on a fascinating corpus of legal documents and letters, written in Bactrian and Arabic in the fourth to eighth centuries ad, which were discovered, edited and translated relatively recently. Scholars of Central Asia have tended to discuss the region's early Islamic history within a politico-military framework based on chronicles and prosopographies written in Arabic and/or adapted into Persian centuries after the Muslim conquests. Such narrative sources describe an ideal state defined by genres of Islamic historiography, and come with the usual menu of distortions, simplifications and exoticisms. The documents under review, on the other hand, were written to serve immediate and practical uses; the evidence they offer is devoid of rhetoric, recording aspects of life and social groupings to which we would otherwise have no access. This paper argues that during the transition to Islamic rule (c. ad 700–771), Bactrian and Islamic administrative systems co-existed, and significantly affected family life and marriage traditions. Specifically, it is suggested that the early ʿAbbāsid tax system eclipsed the age-old practice of fraternal polyandry here: more by default than by design.