This large mixed methods study investigated the effects of the introductory second language acquisition (SLA) course on the beliefs of pre-service teachers (n = 381) enrolled over a three-year period at a state university in California. Pre- to post-course paired sample t-tests that were run on 23 beliefs statements from a widely used survey revealed significant changes in beliefs in several areas including the length of time for acquisition, difficulty of language acquisition, the role of culture, the role of error correction, the importance of grammar, and the efficacy of audiolingual learning strategies. In extensive written post-course explanations, the pre-service teachers attributed their pre-course beliefs to language learning experience in high school and post-course changes in their beliefs to the SLA course content and experiential activities, most notably tutoring an ESL student (the first contact for some teachers). Depth of knowledge was revealed in the teacher comments; this included information that could not have been gleaned from the beliefs surveys alone. The findings, which demonstrated how pre-service teacher beliefs evolve within the context of a professional education course, have practical implications for course design and evaluation in teacher education programs. (Contains 3 tables.)