This thesis examines the relationship between pentecostalism and fundamentalism in the United States from 1906-1943. Of particular interest is the formation of the National Association of Evangelicals, which combined these two movements (along with holiness churches), though their history was marked by dispute. On closer examination, the two groups held an evangelical heritage in common from the nineteenth century. Like a new species that is introduced into a particular ecological context, new religious movements grow and develop in response to their surrounding environment. This study divides pentecostalism’s growth (particularly that of the Assemblies of God and the Church of God [Cleveland, TN]) into three stages: genesis (the introductory period, 1906-1909), adaptation (the formative period, 1910-1924), and retention (the educational period, 1925-1943). Fundamentalism ‘leavened’ pentecostalism by the latter’s adoption of the ‘language’, the ‘content’ and the ‘rhetoric’ of fundamentalist theology, especially through the vehicle of dispensationalism. In the end, the hostility exhibited between them during this period was the result of religious proximity. Pentecostals were a threat to the power structures of fundamentalism by attracting parishioners to its form of revivalism.