This article discusses conversion of pagan Udmurts to Islam in the XIX-XX centuries and their assimilation among the Kazan Tatars and partly among the Bashkirs. In the VII-VIII centuries AD, the Bulgars came to the Volga-Kama region, where the Udmurt tribes lived since ancient times, and they began active contacts with the local aboriginal population, whom they called "ar" (Chuv. ar, tat. Ir - “husband”, “man”) following the Khazars, and the Ar-Udmurts called them "biger". Linguistic, archaeological, anthropological, folklore data indicate active contacts of the Bulgars with the Ar-Udmurts. At the end of the VIII century, especially in the IX century, almost at the same time, when the Bulgars moved to the Volga, a large group of northern Udmurts, for reasons unknown to us, went to the Lower Kama region, to the Volga, where the state of Volga Bulgaria was creating. In the emerging state, mixing of cultural traditions of both groups, their leveling and the creating a new culture, which laid the foundation for the culture of the Volga Bulgaria, is observed. In some areas the Chepets-Ural population prevailed in number over the Bulgars. The process of Islamization and Turkification of the Udmurts was from the middle of the XIX century until the October Revolution of 1917.