When the Sassanid governor embraced Islam in 628, his South Arabian province at least in theory also was considered to be islamized. Relations to northern Arabia were emphasized at that time because of the caliphate’s location in Damascus. Soon thereafter, however, South Arabia defected from the faith again: this apostasy, called ridda, was crushed with military means by the early caliphate. During the subsequent two centuries, however, other deviant orientations and denominations continued to use southern Arabia as a refuge and stronghold: these non-Sunni ways of Islam in southern Arabia (e.g. Ibadiya, Ismailiyya, Zaydiyya) at times provided alternative centres for South Arabian Muslims. The accounts of the ridda and the reaction of the caliphate in early Arab historiography will be interpreted with regard to this issue, complemented by research into the beginnings (and end) of diverse Islamic movements considered as heretical by representatives of Sunni mainstream tendencies. The relevant Arabic sources include important early references to the roles of tribal groups, federations, and their leaders, which will therefore also necessitate a reconsideration and possible redefinition of these terms and their meaning in local regional and linguistic contexts.