The Missionary Sisters of Our Lady of Africa (MSOLA), commonly known as the White Sisters for the color of their habit, were founded in 1869 by Cardinal Charles Lavigerie to evangelize the African female population. In 1894 they were among the first missionary women to enter what is now Tanzania, at that time a protectorate of German East Africa (since 1881), later a British mandate (1920) and finally an independent country (1961). The White Sisters opened the first of several mission stations in Karema (on the shores of Lake Tanganyika) and Ushirombo, along what was then an important slave route from the coast (Bagamoyo) into Congo, and among one of the largest Muslim communities in the region. Islam had been firmly established in the coastal belt of East Africa since the end of the 15th century and continued to flourish during the Omani period (1821-1880). In this paper, I examine the Christian-Muslim interaction that took place at the grassroots level in selected missions of Tanganyika, based on the sources produced by the aforementioned religious congregation. My focus is on the development of this encounter and the sisters' agency to bring about change.