Most histories of East Africa's precolonial interior only give cursory attention to Islam, especially in histories of present-day west-central Tanzania and the eastern Democratic Republic of Congo. Most converts to Islam in this context are usually viewed as 'nominal' Muslims. This article, by contrast, builds on recent scholarship on other regions and time periods that questions the conceptual validity of the 'nominal' Muslim. New converts necessarily questioned their social relationships, ways of living, and ritual practices through the act of conversion. On the shores of Lake Tanganyika, new converts were observable through the act of circumcision, dietary restrictions, abidance by some of Islam's core tenets, and the adoption and adaptation of certain phenomena from East Africa's Indian Ocean coast and islands. Interior populations' conversion to Islam was bound up with broader coast-interior material, cultural, and religious exchanges.