The critical importance of second language (L2) input, output, and interaction in TESOL notwithstanding, the decision between immersive (L2-English only) versus nonimmersive approaches is considered "the most fundamental question facing second language acquisition (SLA) researchers, language teachers, and policymakers" (Macaro, 2014, p. 10). This intervention-based study asked which approach-immersive versus nonimmersive-facilitates acquisition of English as a second language. A longitudinal 15-week study in a U.S. university context contrasted an immersive control condition with a nonimmersive treatment condition at two levels of proficiency (low-intermediate B1 and high-intermediate B2/C1). Examination of student learning outcomes revealed no significant differences between the treatment and control groups in overall course grades, seven measures of paragraph-level writing and three evenly spaced exams in the low-intermediate class, and five measures of genre-based writing and two presentational speaking assignments in the high-intermediate class. Analyses of standardized course evaluations revealed an association between higher student ratings and participation in the treatment groups, specifically regarding the extent to which the researcher-practitioner facilitated discussion. In a reflective journal, the researcher-practitioner noted general fidelity to condition, no qualitative differences in the quality of student performance, and possible differences in the speed of assignment completion and willingness to use the first language.